View source for Some physics related to everyday life


<!--
This is partially just revisiting some high school physics
-->


=Practical questions=

Things that I sort of know but can't recall offhand.

Or happened to have more interesting details.


==Around the microwave oven==
{{stub}}


The most minimal microwave oven would consist of
* a 2.45GHz source (a [https://en.wikipedia.org/wiki/Cavity_magnetron magnetron])
* a chamber that is insulated for that frequency, and
* a way to guide waves from the source to the chamber (a [[waveguide]])


Microwaves deliver energy most efficiently into polarized molecules, creating friction and thereby heat.

There are various molecules we can get to, but the most abundant in the list that is present in most food is water (though also fat and sugar{{verify}}),
and it turns out that at 2.45GHz {{comment|(wavelength of approx 12.2cm, 4.8"))}} we heat primarily water.


So any food with high enough moisture content can be heated in a microwave.

Unaffected materials include most types of glass, paper, ceramic, and many plastics, though there are exceptions.



===Uneven heating===

'''Simpler designs will have some amount of standing waves,'''
implying there are also areas receiving much less energy than others.

This is one reason you may see unevenly warmed food.

The rotating platter is a simple way to even that out, but still isn't perfect.

Microwaves commonly have a {{imagesearch|microwave stirrer|stirrer}}, sort of a metal fan that helps jumbles the waves around, making for less pronounced standing waves. This helps, but isn't perfect either.


Microwave cooking instructions might include stirring (in the 'with a spoon' sense) halfway through,
or letting it sit a while after cooking (to let the heat even out),
depending a little on what kind of food it is.


'''Denser food sees only 2-3cm  (~1 inch) of penetration.'''

If heating something large, the inner parts may need to be cooked via conduction from already-warm parts.
If you want to do that evenly in a microwave, you'd probably turn the power down and wait a lot longer -- but at this point, you might as well use a regular oven, or pan.


'''It also turns out that ice does not absorb energy as easily as water'''.
It melts, but mostly because the energy gets delivered to water on its surface,
so a frozen product will effectively warm slowly at first, then heat faster,
and when something was ''partially'' frozen thing,
that can make some parts be very hot and cooked while others are still frozen.

===Bad ideas===


====Some plastics====

Whether a plastic is microwave safe is not about it getting too hot because of microwaves,
it's about what it might contain and release when hot (mostly because of hot food).


So if it has a microwave label, it's an indicator of what it does ''not'' contain: 
additions known to not less-than-great for you, or even just taste bad.
{{comment|(The main names here seem to be [[BPA]], some [[phthalates]], and some others)}}

====Metal (partly)====

<!--

'''tl;dr''': 

There are entirely safe cases.
The wiry platforms in there are metal, and perfectly fine, because physics.

...but explaining exactly what those physics are, well enough for you to have a simple and conclusive 
basis to know when it is safe, is hard.

...so it' s easier to say "just don't put metal in microwaves".




There are metal things, aside from the wiry platforms,
that are safe, and put in there intentionally.

[https://en.wikipedia.org/wiki/Susceptor Susceptors] are things specifically designed to pick up EM and turn into heat, are designed to be fine.

Say, microwavable Hot Pockets have a sleeve containing aluminium (there are other types).
[https://www.reddit.com/r/AskCulinary/comments/2ef6q0/hot_pockets_have_metal_in_their_wrappers_to_crisp/][https://www.sirane.com/food-packaging-products/microwave-susceptors-crisp-it-range/sira-crisp-crisp-it-films.html]


But these are all specifically designed to be safe in a microwave. 



And some cases may not be much of an issue. 

Say, a spoon sticking out of your tea ''might'' be fine, it might even sink much of the additional heat into the tea before you touch it.

But in general, the issue is that any particular case is an unknown,
unless physics is your hobby or job, and/or have a good memory why or which things are safe.




: '''When and why can it be bad?'''

Metals will act like reflectors {{comment|(which is how the walls act as shields and help cause standing waves)}}, but this isn't the direct issue to most added metals.

Metal can act like antennas, with some shapes and metals being well tuned to microwaves. 
This means they can absorb a good amount of the microwave EM in there.


This can make them quite hot, pretty locally.
Thinner metals will heat much more easily. 

This is probably an issue to your fingers, but not ''necessarily'' beyond that.



The larger issue is that it can set up voltage differences.

Which can be large enough to spark - which is a sudden discharge of energy that the microwave generator may also see, and in some cases can damage it.

Physics-wise, it's mainly cases where the static field induced by the electric field are kept build up to the point they are larger than the dielectric of air.

This is a fairly specific case, because for an arc to happen you needs something closeby at a noticeably different potential. 
It helps if it's focused, i.e. has sharp edges towards that thing of different potential.


Which is why e.g. a fork will do this better than a spoon. And you'll get more disco out of a collection of metal objects near each other.


Smooth metal objects without points may only heat and not spark, and ''may'' accept minimal heat compared to the food that's on it.

Smooth aluminium foil probably fares better than wrinkled, but may still not be a good idea at full power because they're thin.


CDs in a microwave also have aluminium layer (usually), but it's very thin, on the order of 0,0002mm thick so heats very well.  
It seems that it only arcs once some of it has vapourized{{verify}} so that there can be enough potential difference to the metal still in the CD{{verify}}. This is also why it seems to go in steps over a few seconds.


Various spoons and pans won't arc, and non-wrinkled aluminium foil will usually only heat - which is sometimes done intentionally, for example to shield some things from heating, or to focus the heat somewhat (see mention of susceptors above), with the sheet specifically designed to get only moderately warm at usual microwave power.


All this means that some things involving metal may ''accidentally'' be perfectly fine. 

But you probably won't know unless it doesn't kill your microwave.



'''Bad for you or bad for the microwave?'''

You may burn your food or burn your fingers,
but none of that is a lasting issue.

A few cases, like the CD, will put out aluminium vapour, which isn't great for you.


It's more easily bad for the microwave.

It's not even the arcs themselves that are the problem.
Best case it's only somewhat bad for the metal items.


Arcs (whether betwen objects or to the walls) are sudden and involve large currents.
If large and/or frequent enough they can damage the microwave element.

This means the microwave element suddenly deals with much more energy than it was designed for.



Metal touching the side seems to be bad




: '''When it can be safe'''

Basically, whenever there is good reason that the metal won't absorb most of the energy and that it won't arc. 

Given those two, you can even have metalurgic fun in your microwave [http://home.c2i.net/metaphor/mvpage.html] - though you probably want to avoid food in the same one.



https://www.huffpost.com/entry/metal-in-the-microwave-video_n_5642108
-->

====A few foods====

<!--
It seems that some foods with higher mineral content,
in some very specific cases, can have issues like having metal objects,
and it turns out for rather similar reasons.


This works better with more mineral content, with sharp edges, and with specific small distances.

Some vegetables may (apparently mostly those with high mineral content, which is correlated with denser veggies (apparently including green beans, carrots, kale, bell peppers {{verify}}) and it can help when they are kept at specific distance like in a frozen block.

You can try to elicit his effect, e.g. chopping a carrot and placing parts at very specific distances.


It's not likely to happen in most food, because charge cannot build up in most food, with mineral content rately being concentrated, and there usually being a bunch of water and electrolytes around (meaning conduction and thereby dissipation){{verify}}.




You'd think grapes would be fine, because they're mostly water.

Eggs are fine, but poke a hole or the steam buildup will be a messy issue. [https://www.saudereggs.com/blog/can-you-cook-eggs-in-the-microwave/]



https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/appliances-and-thermometers/microwave-ovens-and-food-safety/


-->

====A few ceramics====

<!--
Most ceramics bowls are perfectly fine due to how they are typically made.

There are a few with contents (I think usually water?) that imply the bowls absorb energy instead of the food in them, so they'll be piping hot and the food cold. 


Nothing particularly bad, except in the practical sense. 

Especially if the heat leads to such bowls them breaking, and it was lots of soup.


There is sometimes also metallic paint, which may even arc (the layer will be so thin it is effectively a sharp shape). 

-->

====Empty microwave====

<!--
'''For context:'''

The outer walls of a microwave reflect microwaves, and the dimensions are set up for the microwave wavelength, to reflect and distribute energy, which helps deliver a lot of energy efficiently.

Simpler designs may have standing waves, meaning some spots get much more energy than others.
It is common for microwaves to have a 'stirrer', which basically jumbles the waves around,
because this will cook better.

If there's food in there, or only entirely dehydrated food, little of that energy makes it into that food and turned into heat.


If there is nothing to absorb the energy and become heat, this can be bad for a few reasons,
but proably the largest is that a portion of energy makes it back to the source (the [https://en.wikipedia.org/wiki/Cavity_magnetron magnetron]), heating it up. 

If that is sustained, it may overheat.


Worst case, it leads to thermal runaway, but in practice there is often a [[thermal fuse]] that turns it off, temporarily, or permanently.

Also, with a stirrer this will take much longer.


Exactly how long that takes depends on device and case. 
It may be a few seconds, but more usually it will take a few minutes.



-->

===Other potential problems===

====Sudden boiling liquids====

It is possible for some liquids to [https://en.wikipedia.org/wiki/Superheating  superheat], which for these purposes means their temperature is above their boiling point ''but'' do not look like they are boiling.



This doesn't happen easily.  Pans and heaters avoid this because they heat locally,
and it takes quite-uniform heating to avoid creating local nucleation sites for the bubbles (and movement) we call boiling to start.

It also requires fairly pure water - this isn't easy to do with most tap water.

The movement involved in picking it may be enough to shift things and start the boiling,
as may adding something at a different temperature (spoon, another ingredient).


<!--
You can avoid this in a few ways:
* stirring the liquid may be enough
* adding another ingredient tends to help (adds ingredient and air)
-->

===Some more notes===

: '''Isolation'''

Microwave ovens are well isolated by the internal metal walls, by the door's grated metal sheet (because of the microwave wavelength), as well as the chassis.

The little microwave EM that gets out is several orders of magnitude weaker than what's happening inside; it's milliwatts rather than the kilowatt-ish rating the microwave probably has, and rather smaller than what is known to be able to hurt us, or even be perceptible as heating.

Microwaves won't get out the air exhaust, largely because of the wavelength involved combined with the designed physical characteristics.



Pacemakers could ''in theory'' be affected, with the small wires acting as antennae. It can't hurt to be overly careful, and you might want to avoid a job at a microwave radar site, but microwave ovens are quite unlikely to cause problems.



Microwave frequencies mean this is non-ionizing EM radiation, meaning that they won't destroy cells, just heat them (comparable to cell phones).

You wouldn't want to stick body parts inside of an over, because it will heat you just as well as it heats food, which working organs won't like. But the little energy that escapes is unlikely to be even felt.

==Ions, Ionizing radiation==
<!--

So, I couldn't recall all the details offhand either, 
and some physics and chemistry is stuff you deal with in everyday things like batteries and food, should probably be better known,
so I figured this one is worth writing out.

-->
===Ions===

<!--

'''What does it mean to say a thing is an ion, or ionized?'''

You may remember from school that an ion is an atom or molecule that has an electron too little or too many, compared to what it would normally have.
: An electron more than it needs makes them '''negative ions''' (because electrons have negative charge), e.g. '''anions'''
: An electron short makes them '''positive ions''', a.k.a. '''cations'''



Yay definition, but in isolation that means very little.

Are ions a tangible property, like e.g. electrostatic charge, that could do something like zap you after shuffling over a carpet,
or just some low level detail like that there are multiple bond types keeping solids together, that you can name but not particularly change?

Are they healthy, unhealthy, neutral?

Are they a common natural thing or a rare man-made thing?

What sources of ions are there, and what amount does each of them output?

If it's just an electron thing, do they discharge? And if not, why not?


tl;dr:
: ions, of any sort, are fairly reactive

: whether that's bad, neutral, or even required for something, depends largely on amount, and where they are
:: high concentrations may not be great
:: yet there are many cases where the concentration is quite low

: there's quite a few natural sources, and a few synthetic ones



-->
====Where do ions appear?====
<!--
A few different things:



''Various chemistry'' 


'''In liquids''', "electrolyte" means "medium containing enough ions to be electrically conductive through them",

In practice, a lot of electrolytes are salts, acids, bases, and typically in water.
* acids are ionic compounds involving hydrogen ions (H+)
: (pH is a measure of the amount of active H+ ions)

* bases are ionic compounds involving hydroxide or oxide ions (OH- or O2-) 

* salts are a balanced combination of cations and anions.


Electrolytes include a bunch of natural liquids, and a bunch of synthetic ones - e.g. most batteries.


'''For solids''', [https://en.wikipedia.org/wiki/Ionic_compound ionic compounds] refer to are a combination of positive and negative ion, held together by what we call ionic bonding.  {{comment|(contrasted with molecules, which are things held by [https://en.wikipedia.org/wiki/Covalent_bond covalent bonds])}}

This includes the undissolved forms of those same acids, bases, and salts.
Solid ionic compounds tend to be regular, sit in a lattice structure, i.e. crystallize.


In practice, we often think of acids and bases as a liquid (also because you can't have pH without it being in water).
They can also be liquid themselves, but usually not at temperatures we care about (e.g. table salt at 801 °C).
 

In liquid form or in solution, all of these ''could'' be fairly reactive, 
because it makes more than a few of those ions actually available and movable.


For a more concrete example, take (electrochemical) batteries. [https://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery Lead-acid batteries] are named for its sulfuric acid, H<sub>2</sub>SO<sub>4</sub>, which can split into H<sup>+</sup> and HSO<sub>4</sub><sup>-</sup> ions in solution.
Basically, that battery is charged when those ions in solution ready to react, and flat once it's all reacted to the plates (no longer ions), and charging it is kicking them back to be ions in solution.



''Some biology'' 

We say we're water, and most of that is electrolyes - a handful of specific 

In particular [https://en.wikipedia.org/wiki/Ion_transporter ion pump]s,
which we need for our brain, muscles, and cells in general to work - neurons communicate with each other by pumping ions from one to another.
Salt is also required. 

Yet the amounts of ions involved in this are ''minuscule'' compared to most other things on this list.
They are harmless because of that small amount, because their generation is well controlled,
and because they react fairly soon with what they are ''intended'' to react with.




'''Everyday sources'''

'''Waterfalls, crashing waves, showers''', in fairly small numbers.

'''Plants''', in tiny numbers



(Note that the actual ions differ between Corona, natural, Lenard sources)


'''Weather'' - ''roughly'', moving atmosphere collects charge, slowly but in bulk, and during release which will get rebalanced in the form of lightning - and it is that lightning involving so much energy means it produces a whole bunch of ions in the air.

Large electrical discharges ''in general'' can ionize air near it (also related to containing UV, as e.g. arc welding, power lines, transformers, and some other high-voltage equipment).
These are often more relevant to us than lightning, in that they are often closer to us.


Strong magnetic fields can in specific cases do the same [citation needed].


''Extreme heat'' can ionize, but that requires "places where plasma lives" kind of hot
and roughly nothing natural near us (and little man-made) sustains heat 
to do this much of that (like e.g. a plasma chamber would).

The sun does involve hydrogen ions and other ions, and [https://en.wikipedia.org/wiki/Solar_wind solar wind] contains a small amount of them.



'''Intentional sources'''

There are various processes/designs that involve, or act as, [https://en.wikipedia.org/wiki/Ion_source ion sources]. 

Some intentional and more concentrated ones include
: spectrometers
: particle accelerators, 
: ion implanters, 
: ion engines, 
: air ionizers (usually corona discharge, or UV)

Most of these are also closed metal boxes, for a few different details we'll get to later.




'''Ionizing radiation''' is not about being ions itself, 
but something that involves enough energy to break something else into into ions.

At small scale this is more or less irrelevant.

For example, [[cosmic rays]] (basically: small matter that happens to go at nearly the speed of light).
are ionizing, but also arrive rarely enough that they barely matter to us.

A subset of UV is ionizing, but human-made sources will be low doses and have lots of warning labels,
and the UV that comes from space is mostly filtered absorbed (by ozone - this is why we made a fuss about the ozone layer)
: which is why part of UV can be used as a sterilizer


The real issue, though, is that a lot of other sources of ionizing radiation are immediately good at doing this a ''lot''.
Particularly radioactive sources come to mind.





'''Can solids have ions?'''

Solids can ''can'' involve ions, though in most cases it's just a few on the surface.

Ionic compounds, like salts, will break into constituent ions more easily than molecules,
basically because molecular bonds take more to make or break.

Ionic compounds don't break without reason, and therefore also not much by themselves,
but they still work out as more reactive than most molecules.



'''Can metals be ions?'''

Metals are grid-like structures, and it works out that the sum of electrons that make the whole clump of metal work out as neutral can shift fairly easily within it.

That's also how they carry charge more easily than most other materials. (a semiconductor is sort of similar in that it's still about electrons, but a lot more constrained, which is why we talk about moving them around through electron holes)

It's more ''practically meaningful'' to refer to the entire thing as charged (rather than ions),
in part just because the definition we use says electrons associated with specific atoms. 


'''Does electricity involve ions?'''

Electric current itself is the flow of electric charge, of electrons.

Pushing electrons through a metal is the most efficient and most common way, 
But something has to push on those electrons. 

Ions are not involved in either described things, so no.


You could say that in electrochemical cells (everyday batteries) it is the move of ions that does that pushing,
so in that case, a qualified yes.



'''Is static electricity ions?'''

Electrostatic charge are electrons, a buildup of electrical charge that hasn't found a reason to move on yet.

That practically means it's about surface, of larger objects.
Buildup itself is moving them from one overall solid to another - due to friction, or rather, the separation of two surfaces.

It builds up equally on conductive or isolating solids - though when they happen to be grounded, it is only conductive solids that provide an easy path to be discharged.

So no.



https://www.quora.com/If-static-electricity-is-caused-by-the-temporary-loss-of-electrons-is-it-easier-to-get-shocked-by-more-reactive-elements-where-the-electrons-are-more-easily-lost

https://www.scienceabc.com/pure-sciences/why-does-static-charge-build-up.html

https://www.americanscientist.org/article/what-creates-static-electricity



'''Is plasma ions?'''

Yes, plasma is by nature ionized, being the state of having so much energy that electrons become disassociated.

We don't see it much because it requires very high charge or temperature. 
Which is also why it's bad for us in general, not because they're ions.


'''So are gases ionized? Should we worry this happens easily?'''

A plasma is a gas where much of it is ionized,
so it's a good question.

But no, gases are still together.

At most a small amount of any gas might be ionized
often by abundant energy, 
but ions react, so to have ''all'' of something be ions needs a large amount of probably-sustained energy. Think 'in plasma chamber' or 'in the sun'

Being on fire would technically also 
make all of you ions, but small parts at a time.




'''Does air contain ions?'''

Yes, and atmospheric ions are arguably the ones you can have the longest discussion about.

Atmospheric ions will find usually something to react with,
particularly if there are both positive and negative ions around.

This means they're generally decreasing in number, 
so there are generally few sticking around,
so unless you're close to a constant source of them, they are not a worry.


'''Do salt lamps produce ions?'''



-->

====So what's the difference between ions and electric charge?====
<!--
Charge is the property, one that can happen at various scales.


For example, [[electrostatic]] buildup is charge that comes from moving a large amount of electrons
: For example, a big chunk of metal can be charged, as can non-conductors.
: and while electrostatic discharge describes those electrons leaving suddenly, that still doesn't mean that chunk of metal was any more reactive


Ions can be seen as charge that comes from the nature of individual atoms or molecules {{comment|(at subatomic scale - protons are also relevant, but those are pretty fixed, whereas electrons are the things that you can move around, so they end up being the main thing that makes ''each'' atom/molecule neutral or not.)}}


As to reactiveness, it also helps that ions often imply are often in fluids.

Pragmatically, ions when it's atoms/molecules in liquids, suspensions (and arguably a few solids), gases, and plasma -- mostly because the reactive things are readily available everywhere - also meaning a lot of effective surface area.



----


We need to talk about the things that make up atoms. The lowest relevant fundaments are basic subatomic particles,
namely the combination of electrons (has negative charge always), a proton (positive charge always), a neutron (no charge).

These sit together as specific atoms, and atoms are typically balanced.
For example, carbon has 6 electrons and 6 protons (and 6 neutrons) and balances out.

If there's an unbalance, it's charged, and when we refer to atoms or molecules being changed, we call them ions.

So ionization is charge at atom/molecule level, whereas at lower level you talk about the reasons they are,
and at higher levels you tend to just fall back to 'charged' because it's a broader and usually more correct term.

A piece of plastic or cloud is charged - but due to the nature of ''how'', charged is
(while calling each of these ionized is more specific and sometimes wrong)

There are subtleties, though. A liquid or gas can more easily contain positive and negative ions in different places,


----

You can make most atoms into ions by whatever kind of work moves electrons.

An ion is an atom or a group of atoms that is charged. It becomes charged by losing one or more electrons or gaining one or more electrons.


Charge itself is a property of the sub-atomic particles themselves. Specifically, each electron in an atom has a single negative charge, and each proton has a single positive charge.
Neutrons have no charge, so don't matter here.

An atom or group of atoms is neutral or uncharged unless it is ionised.



So for example, a carbon atom has a charge of 0 and is not an ion. Yet within it, it contains six positively charged protons and six negatively charged electrons, and approximately six neutrons. The 6 positives and the 6 negatives make a total of zero, so uncharged.


-->
====Why can ions be good or bad?====
<!--

Chemically, an ion wants to return to neutral charge, meaning they are fundamentally quite reactive.


Anything reactive that is present in large enough numbers, is potentially bad.


Anything that can selectively damage your DNA, and do it enough for actual mistakes, is called cancer.






: types of ions

A lot of matter ''could'' be ionized, but in most cases, direct physical context determines how much it matters.





Acids and bases could be said to be bad because ions, 
which is both true, but we can also just say they're reactive chemicals.


Ionizing radiation can be bad because a source 

Plasma is bad not so much because ions, but because it's so damn not.

These are also solids or liquids, so mostly not something we 
have contact with.



So in terms of health, we mostly care about 
What we care more about are '''atmospheric ions''' / '''air ions''',
which are ionic atoms/compounds that and linger in the air.

As the natural generation of these is low,  and ions in the air will easily react within seconds or minutes, 
the concentration of these is low.




: positive and negative ions

You'ld think these would be equally disruptive, and you'd be sort of right,
but - turns out there's a lot of study done of this - it turns out negative ions 

A small dose of ions can be enough to damage bacteria,
which in theory can help against germs and allergies, 
though studies suggest little effect at levels safe for humans.


Suggested effects
* boosting the immune system
* better oxygen delivery to the brain
* better bloodflow
* higher serotonin production
* higher melatonin production

...which would have the effects of 
higher awareness, 
less depression, 
less stress
and better sleep.



There are many studies that suggest possible biological effects,
and some that find conflicting effects,
but fewer that have studied them in detail.

Which currently seems to mostly amount to 
"nothing conclusive, because not well studied yet",
and effects may have confounding causes.

Given this, it's overstated at best.



https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6213340/




Shu-Ye Jiang et al. (2018) "{{search|Negative Air Ions and Their Effects on Human Health and Air Quality Improvement}}"
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6213340/]




: '''so what's the difference between ions and radicals?'''




: Are ions dangerous?

Like so many things, it's not about what, but how much.


Even ionizing radiation isn't bad in low doses - which is good because there is a natural low background level of it.


but a practical issue is that probably you won't notice it,
so you may as well try to avoid it in all ways.

The relationship between amount of radiation and danger is not linear.

In particular, there is basically an amount of damage per year that your body can deal with.
This is why it's okay to fly, eat bananas, and exist (our biology needs potassium - which is ever so slightly radioactive, same reason bananas are).



Also, the problem isn't that there ''are'' ions -- you wouldn't function without them[https://en.wikipedia.org/wiki/Ion_channel] and they can be potentially useful (see e.g. [https://en.wikipedia.org/wiki/Air_ioniser air ionizers].


In most cases ions are not dangerous,
not because they're not reactive, but because most any source of them is tiny, 
or controlled.

Consider the case in biology: aside from being necessary,
its the source and use is small, controlled and selective.



Any reason for a large amount of ions in tends to be a large thing, not selective, probably not controlled, possibly focused/concentrated in a small place.

Most sources of ionizing radiation, for example.


The problem is that many sources are completely indiscriminant ''what'' is being ionized,

It will break apart any molecules, and you particularly care when these molecules are ''you'',
more so because biology is sort of fragile at a microscopic level.

In particular breaking DNA is not a good thing - it breaks biology at its fundaments, namely the messaging between its parts. (bodies can deal with a small amount of such damage because they have to, but not a lot of it)



Sometimes they're indirectly bad.
Lightning involves a lot of ionization.
Being struck by lightning may not break your DNA, but you will still have a bad time.







'''Product-wise'''


Air ionizers
: ionize the larger particles in the air that passes by, which makes it more likely to stick to the purifier, each other, and the surfaces near it.
:: sometimes combined with a mechanical filter and/or fan - just relocating it from air to surfaces is a neat trick, but not as useful as removing it
:: used at larger scale around e.g. waste incineration[https://en.wikipedia.org/wiki/Electrostatic_precipitator]
: not dangerous in itself - much too small an amount of ionisation and it's not near you {{verify}}
: most air ionizer products are much too weak to live up to advertised promises, but they still work
: most methods will also produce ozone (more so if actually strong enough), which has to be dealt with because releasing it ''consistently'' is unhealthy



'''Negative ion lamps'''
: Don't do anything.
: They're usually pretty, though.


'''Negative ion bracelets'''
: Ignoring the ones that are snake oil and do nothing...
: If they do put out ions, they fundamentally need an energy input. 
: That amounts to wearing a radioactive material. 
:: weak, but not weak enough to not matter at all.
: depending on the construction, you may basically be spreading it around
and breathing it - where it can do more damage than it could on your skin.


'''Negative ion powders'''
: basically a worse case of the previous point












: '''Do cell phones emit ionizing radiation?'''

No. The [[EM spectrum]] starts to be ionizing at higher frequencies, somewhere in the middle of UV. UV-C, x-
Everything below that (visible light, infrared, microwave, radio) cannot and the worst you can do with that is get you warm.








https://www.youtube.com/watch?v=C7TwBUxxIC0


P Kolarz et al. (2012) "Characterization of ions at Alpine waterfalls"


-->

====Negative ion generator====
<!--
A negative ion generator, e.g. the household item, is often a Cockcroft-Walton design:
* a capacitive voltage multiplier
* needles
* resistors as a current-limiting safety device


https://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_generator

https://www.youtube.com/watch?v=ep3D_LC2UzU

https://www.youtube.com/watch?v=B4RTrjABi3M





There are more negative ion produts, from bracelets to eyemasks.

Do they do nothing?

Ideally, yes. 

A few of them actually live up to the name -- ''by being radioactive''.

-->

===So what is an ionizer?===
<!--

As a device? That varies. 


These are usually sold as air purifiers.


High voltage device that puts an electrostatic charge into the air.


In many designs this ''also'' produces ozone.





-->

===Ionizing radiation===
<!--

Ionizing radiation doesn't ''come'' from ions, it ''causes'' ions - among other things.


Ionizing radiation is ''anything'' (particles ''or'' electromagnetism)
that has enough energy to (indiscriminantly) free an electron from another molecule.

Anything that that can make molecules into ions.


And like mentioned before, that's bad when that happens to be any of your molecules, indiscriminantly.

The point isn't that they can at all,
but that sources ionizing radiation tend to have a ''lot'' of it.



Due to the way atoms do energy exchange (see [[quanta]]),
this requires each ''individual'' interaction being able to do so,
which is why this is something that starts happening at a pretty specific spot in the EM spectrum.


Almost all stuff humans consider useful - heat, solar power, visible light, lasers, most radio waves - sit below that, in the range that isn't energetic enough to be ionizing.

Which isn't surprising, because 'damaging' and 'useful' are generally exclusive.




Anything with less energy will at most ''excite'' the electron but not help it escape. 

In large doses you'll get warm, in usual doses you won't notice it.

This is roughly how how a microwave heats water - a high-power, focused, short range emitter
at a specific microwave frequency (that of the resonant frequency of water, 2.45GHz).

It could hurt and damage you, but only by locally boiling your water and doing so inside your cells.


But that's true of light as well. Find a good enough lens and you'll burn.

The only difference is the amount of focus and penetration. 
Like a microwave oven has maybe an inch of penetrating power (and ''only'' about that, which is why you can't cook very large things in the microwave).


Similarly, if you find a high-power high-frequency radio transmitter, climb it, and and standing in front of the right antenna long enough, you'll get very warm.




'''When does it become bad'''

The energy to break molecular bonds is bad when it bothers you.


It's harder to manage that for things that are invisible.

This is also where the sensible yet overly cautious fear comes from - if some radiation is bad and some of it isn't, how do we even tell?


Now, different molecular bonds break at different energies,
so for something energetic enough to be ionizing, most figures in the range of 10eV to 30eV.

Adding some extra safety margin, let's say we want to avoid anything over ~5 eV.

There's more than one way to do this.

One is an electron going at ~1300 km/s, but that's not common unless you're in a vacuum and/or making them specifically. 

So practically, we care about electromagnetism when when their quanta carry that much energy, which happens above around ~800THz, in the UV range.

{{comment|(For reference, 700THz is still optical, namely visible purple, and UVA starts at ~750THz)}}


So all the lower end of the spectrum - radio, microwave, infrared, visible spectrum, the first parts of ultraviolet, is non-ionizing. 

It can deliver heat, have photochemical effects, microwaves are targeted at water molecules.  
Cell phones may heat you to a just-measurable degree, but you don't need to 

But none of it ionizes.


Ionizing:
* high-energy end of the EM spectrum: higher ultraviolet, X-rays, gamma rays
: yes, a small part of sunlight is ionizing. That's why we made a big deal about our ozone layer (which blocks it) being weak.
: gamma rays are produced by some radioactive elements
: And a big first-wave thing in a nuclear detonation, but let's assume that if we do that one for real we probably fucked ourselves anyway

* alpha particles, which include two protons and two neutrons (essentially a high speed helium atom)
: They don't penetrate well at all, so only ionize on the surface, but that's bad enough.
: Note that radiation suits work well against alpha radiation

* beta particles, which functionally are high-energy electrons and positrons {{verify}}. Limited penetration





This means that even if you stick a fork in the microwave door sensor and leave it open
and stick your face in front, you will mostly just warm your face
(but don't do this, if there happens to be a focal point on e.g. your eyes that could damage them permanently)



Ozone is relevant because its absorption sits right in the range where it is relevant to UV,
mostly letting through UVA, blocking most blocking UVB and almost all UVC.
This is why the ozone later, a.k.a. "having a bunch of ozone in our atmoshere", is a useful thing.



Full-body security scanners are typically "millimeter wave imaging", which is well below even a 1THz,
in the microwave range.

There exist X-ray based models {{verify}}, but they are not in typical use {{verify}}
-->

===Nuclear radiation===
<!--

You can get very technical, but what we usually mean with '''radioactive materials''' are atoms that are naturally unstable and which, 
in the process of become more stable (= radioactive decay), emit nuclear radiation.


In the widest sense, '''nuclear radiation''' is quite literal, referring to pretty much ''anything'' that flies from an atom's nucleus because of that:
: alpha particles,
: beta particles,
: gamma radiation,
: conversion electrons,
: neutrinos,
: plain light,
: and a few others.

When we say nuclear radiation, we don't mean that full list, because a lot of it is tiny amounts and/or harmless.

We mean the stuff we ''don't'' like, specifically the ionizing part of that list, in particular alpha particles, beta particles, and gamma rays. Roughly ordered from less to more penetration/damage:

* alpha radiation
: two protons and two neutrons
: ...basically helium-4 without its electrons, and if it finds electrons, it becomes plain Helium-4
: while highly ionizing, they don't get very far, barely penetrating skin, not getting too far in air.
: the "a piece of paper will stop it" diagram you find is ''slightly'' optimistic. It'd have to be thick paper.

* beta radiation
: high-speed electron or positron

* gamma radiation
: is high-energy EM. It arrives, does its damage, and is gone
: penetrate further than alpha and beta
:: e.g. you need a few cm of lead to stop it




: Where does nuclear radiation come from?

''Most'' nuclear radiation out there comes from radioactive decay. 

As in, there are few immediate other sources. Acceleration experiments are one.



: Why/how does radioactivity linger?

'Nuclear contamination' has, mainly through abuse in media, become a bit vague.

We should make a clear distinction between exposure and contamination:


'''Radiation exposure''' means you've been near ionizing radiation.
: Gamma radiation is high energy EM, only.
:: so it arrives, does its damage (creating ions and breaking molecular bonds), but cannot linger.
: it can come completely without contamination (whether it does depends on the source)


'''Nuclear contamination''' means that radioactive material in some form is now sitting on a larger object
: we care because it will cause some radiation exposure as long as it has not completely decayed
: we care particularly when that larger object is people, things near people
: we care to prevent rather than fix, because contamination is annoying to clean up so we want to prevent it as much as possible

: This is often due to small dust/powder, or something suspended in liquid
:: fine dust is why decontamination is useful - it's basically a thorough shower to wash these things off so they don't do ''further'' damage to you, and don't get spread around to others.



Around living organisms, there is also a useful distinction between internal and external contamination

'''External contamination''' is that what settled on you.
It tends to do damage near the skin.
It spreads easily enough, because it rubs off and around as much as anything else,
but also fairly easy enough to remove via a decontamination-type shower (which is mostly just a regular water shower from more directions. With some footnotes - like heavy scrubbing is a ''bad'' idea because of small abrasions that give it more texture to settle in).

'''Internal contamination''' means that which is somehow inside you - abrasions your skin, or ingested, or otherwise absorbed.
It will do damage while it's there, so tends to do more damage than external contamination typically does.




More practically
* It's not radiation itself that lingers. 
: It's radioactive material that hasn't decayed into negligibility yet that lingers.

* 'Radioactive groundwater' doesn't mean the water itself is radioactive, but that is nuclear contamination in it - some radioactive material.

* Nuclear contamination is also why a nuclear bomb, or nuclear meltdown, ''stays'' bad for a while.
Both cases are likely to produce and spread ''very small pieces'' of radioactive material, the fissile material itself, 
and often quite a bit of it.




'''On gamma radiation'''

Gamma radiation keeps on being produced during decay, but you can file that one under "the general issue of being around radioactive materials."

Most of the gamma radiation that a ''bomb'' will give off comes from the initial blast{{verify}},
so if by luck or foreknowledge you can shield that one event, then you've avoided that part of what is essentially one-time damage.


To block gamma radiation you need very dense material (e.g. lead, and then fairly thick), and/or a lot of it (e.g. very thick concrete, a large amount of water))}}, which is why a desk as cover is nonsense.

Fallout shelters are built to allow people to wait until such emitted gamma radiation levels are hundreds or thousands times lower which, depending on the initial levels, is a very reasonable thing to do.


At the same time, gamma radiation is EM, so speed of light, so reacting to a flash isn't fast enough.

Which is why duck and cover is mostly nonsense. In the context of the time it might still help in other ways -- like shield against the window's glass that is likely to shatter from the blast, and give people a sense of controllability rather than panic.


'''On alpha and beta radiation'''

Still, the larger issue is the long term.

The bomb itself had a lot of fissile material and, being a bomb, puts it all over the place.
There is a whole mess of different isotopes, and they go mainly into the mushroom cloud.
If it's an explosion in the air, then it's more likely to be diluted over a large area.

Nuclear fallout is a term everyone knows because it's pretty obvious that it has to go ''somewhere'', and right now it's on the wind.


When most of the products have a low enough half-life to matter over the weeks it takes to settle,
and the rest is diluted over a large area, negative effects are limited.



Technicalities:
* any atom currently near ionizing sources can re-emit ionizing radiation
: in that some can ''become'' radioactive from receiving enough (high-energy/ionizing?) radiation to transmute their nucleus into one of its radioactive isotopes
: ...but it only has this energy only very briefly{{verify}}, so if it's non-contaminating exposure, this is more of an consercation-of-energy thing with no net effect, and basically irrelevant
: also, not all nuclear radiation does this {{verify}}


* a tiny fraction of matter everywhere gets enough energy to be radioactive{{verify}}
: ...but in amounts that are entirely negligible

* basically everything around us is ''very mildly'' radioactive - at a level thousands of times weaker than would ever bother our biology.


* '''nuclear waste''' is the fact that, after releasing energy in a way we find useful, the now-different material is also radioactive and less useful to us
: there are usually ways to reuse that waste. It's just not as economical as finding more of the original resource




* radiation is not contagious. Unless the source of it is. To tangent off, you could down radioactive 'energy' drink that's an alpha particle source, and destroy your own biology relatively soon in the process, but if that's all of it you would actually contain most of it. Yes, your excretions will have it and contaminate the waste processing, but from one person it wouldn't even be that much.

-->

===How much ionizing radiation is normal, how much is bad?===
<!--


Your body can deal with damage at low rates - remember that background radiation is primarily natural and has always been there (though certainly varied over time).

This is ''not'' a linear relationship, and there's no singular threshold, but ballpark, you should be mostly fine if you stay under millisieverts per day (note that's ''well'' under any of the above)



'''Everyday levels'''

Note:
: using milli everywhere to avoid milli/micro confusion)
: these are orders of magnitude, meaning they have the right amount of zeroes, but can can vary a few factors depending on situation



orders of magnitude:
: Background radiation is roughly 0.01 milli-sievert per day (4 milli-Sievert per year)
: Eating a banana adds 0.0001 milli-Sievert 
: A dental X-ray is 0.005 milli-Sievert 
: A transatlantic flight might be 0.01 milli-Sievert 
: A chest X-ray might be 0.02 milli-Sievert 
: indoor trapped Radon can add on the order of 1 milli-sievert (per year?{{verify}})
: A head CT scan might be 2 milli-Sievert 
: the average exposure to medial is 0.1 milli-Sievert


'''The realm of danger'''

radiation workers maximum one-year dose is 50 milli-sieverts (except in some emergency positions).

100+ milli-Sievert per year is clearly linked to increased risk of cancer.

Particularly when delivered in a short term:
: 400 milli-Sievert  is radiation poisoning,
: 2000 milli-Sievert is severe radiation poisoning, 
: 4000 milli-Sievert is likely to lead to death,
: 8000 milli-Sievert is almost certain death.




https://xkcd.com/radiation/

https://ec.europa.eu/health/scientific_committees/emerging/docs/scenihr_o_036.pdf



For context, 
: '''Sievert''' (Sv) is a unit of dose of ionizing radiation, one more tuned to radiation's effect on biology like, you know, ''us''. 

: It's a SI derived unit, equivalent to 100 rem (https://en.wikipedia.org/wiki/Roentgen_equivalent_man rem] previously served the same 'effect on biology' function)

: there's also [https://en.wikipedia.org/wiki/Gray_(unit) Gray] (Gy) is also roughly equivalent to a Sievert for X-ray and Gamma, but not for higher energy such as neutrons and alpha radiation


-->
====Electrolysis====
<!--
Electrolysis is one way to separate an ionic compound into its ions
-->

===See also===
<!--
https://en.wikipedia.org/wiki/Ionizing_radiation

http://www.epa.gov/radiation/understand/ionize_nonionize.html
https://en.wikipedia.org/wiki/Non-ionizing_radiation
http://www.epa.gov/radiation/understand/alpha.html
http://www.epa.gov/radiation/understand/beta.html

-->

=EM spectrum=

<!--
There's some bad influence from habit here.

You coul

Light is EM. Our eyes just happen to be sensitive to a ''very narrow'' part of EM.


Radio is EM. It's a very specific way 

You can't listen to 
-->


==Quick(ish) reference==
{{stub}}

Note: There are a lot of infographics you can find that are '''much nicer than the below'''.
(This is here for an experiment which I have not nearly finished yet)


Perhaps most broadly, you can see the spectrum as
* radio (arguably including microwave)
* light (IR, visible, UV)
* harmful radiation (UV, X-ray, Gamma)


Or, a little more detailed:

{| class=wikitable style="text-align:center; font-size:12px; float:center; margin:2px"
|-
! Name 
|| Wavelength 
|| Frequency (Hz) 
|| Photon energy (eV) 
|| grouping 
|| [[ionizing radiation|ionizing]] or not?
|| blocked by atmosphere?
|| Notes

|- 
| Radio 
|| 1 meter – 100,000&nbsp;km 
|| 300&nbsp;MHz – 3&nbsp;Hz 
|| 1.24&nbsp;μeV – 12.4&nbsp;feV 
|rowspan="2"|thermal [1]
|rowspan="4"|no
|rowspan="2"|lower wave radio is blocked, higher end not
| under ~30kHz (1km) there are only a few niche uses that can barely be called radio

|- 
| Microwave 
|| 1&nbsp;mm – 1 meter 
|| 300&nbsp;GHz – 300&nbsp;MHz 
|| 1.24&nbsp;meV – 1.24&nbsp;μeV
|| many applications are much like classical radio

|-
| Infrared 
|| 700&nbsp;nm – 1&nbsp;mm 
|| 430&nbsp;THz – 300&nbsp;GHz 
|| 1.7&nbsp;eV – 1.24&nbsp;meV 
|| thermal/optical
|| mostly blocked
|| (the thermal/optical split is sort of arbitrary)
|-
| Visible 
|| 400&nbsp;nm–700&nbsp;nm 
|| 790&nbsp;THz – 430&nbsp;THz 
|| 3.3&nbsp;eV – 1.7&nbsp;eV 
|| optical
|| mostly passed
|| 
|-
| Ultraviolet 
|| 10&nbsp;nm – 400&nbsp;nm 
|| 30&nbsp;PHz – 790&nbsp;THz 
|| 124&nbsp;eV – 3.3&nbsp;eV
|rowspan="5"|bond breaking
|rowspan="5"| yup
|rowspan="3"| mostly blocked
| bond breaking starts in the higher range of UV
|-
| X-ray 
|| 0.01&nbsp;nm – 10&nbsp;nm 
|| 30&nbsp;EHz – 30&nbsp;PHz 
|| 124&nbsp;keV – 124 eV&nbsp;
| damages life
|-
| Gamma ray 
|| less than 0.01&nbsp;nm 
|| more than 30&nbsp;EHz 
|| more than 124&nbsp;keV 
| damages life
|}

[1] - 'thermal' is the classical name, named not for feeling warm, but the mechanism: [https://en.wikipedia.org/wiki/Thermal_radiation thermal radiation] is EM that originates in thermal motion of matter. This is much wider than heat/infrared. But yes, you ''can'' warm things with much of this range if you're really trying. 'Thermal' seems contrasted with 'optical' but that's a fuzzy distinction too.





'''In a little more detail:'''
* '''Below 3kHz''' or so
:: static field
:: power-line
:: easily influenced by atmospheric changes, and by noise from devices, and the antennas would have to be impractically large
:: ...so doesn't have a lot of radio-like uses
<!--
* there's a lot of electronics that produce noise, by switching and having wires that will act as antennae (e.g. particularly where more current is involved, e.g. LED dimmers, induction stoves, mobile phones, etc)
: generally
:: noise that falls away in a few meters because they're not very high energy, often due to regulations
:: up to a few kHz or MHz due to what they do
: this is one a few reasons there are a few transmissive uses below a few dozen kHz (low bandwidth is another)
-->

* '''Radio wave''' is a broad part of the spectrum
: most broadly 'Radio waves' is 3Hz .. 300 MHz (100000km .. 1m)
:: and if you consider microwave part of it, up to 300GHz (1mm).
: as just mentioned, there are few practical radio-like uses below a dozen or so kHz
: a lot of the Radio/TV frequency band names come from incremental amounts of use during the era that radio and TV were in active development
: microwave itself is arguably just the next "well, turns waves this small are ''also'' usable for similar uses" (they were initially harder to generate) 
: Examples:
:: TV, AM and FM radio typically within 50MHz..1500MHz (60cm .. 20cm)
:: mobile phones are largely at a few GHz (order of 10 cm)
:: frequency choice is also influenced by practicality of antenna size
<!--
: things with wider bandwidth requirements tends to be modulated up more, just because the lower frequencies would be 
: speech and audio basically needs to be above dozens of kHz (but are usually higher), video in MHz or more range, because that's the order of data you have.
-->

: Radar (and radiolocation) is more a technique than a frequency
:: there are radar-''like'' uses between 3Hz and 100GHz {{comment|(see e.g. SUMMARY OF RADAR OPERATIONS in [https://www.ntia.doc.gov/files/ntia/publications/ntia00-40.pdf], though their practical use is fairly specific to range)}}
:: though a lot of it is on the order of hundreds of MHz (longer range) to dozens of GHz (shorter range){{verify}}


* '''the terahertz gap'''
: ''roughly'' 0.3cm .. 30um, 0.1 THz to maybe 10 THz 
: so roughly "the bit between microwave and lower infrared"
: called this because generation and detection here is inefficient for practical reasons, and not in mass production
: ''sub-mm wave'', is an often narrower part of this, used in astronomy


* '''Infrared''', 1mm .. 0.75um, 0.3THz..400 THz. There is more than one sub-categorisation of IR, but we often go roughly by:
:: FIR (far infrared) (1mm..14um, ~0.3THz..20THz)
:: IR-C (14um..3um, ~20..100 THz) <!--e.g. passive thermal imaging-->
:: IR-B (3um..1.4um, ~100..200THz), 
:: IR-A (1.4um..0.75um, ~200..400THz)<!--, e.g. image intensifiers-->
: Infrared is called thermal, but  
:: that's partly because it's part of the (much larger) range of thermal radiation. 
:: If you meant actual warmness
::: it's only perhaps half of e.g. the sun's thermal delivery
::: only part of IR is particularly warm, or practical to warm people - preferably far, but mid also works <!--. Examples:
:::: those glowing porch heaters are mostly MIR, apparently because it's easier to throw out a lot of heat with this
:::: while inside comfort heaters are mostly FIR, apparently because it's gentler and a little more pentrative, and more practical/efficient indoor-->


* '''Optical'''
:: ~700nm, 450THz - red 
:: ~600nm, 500THz - orange 
:: ~580nm, 520THz - yellow 
:: ~530nm, 560THz - green 
:: ~480nm, 650THz - blue 
:: ~450nm, 700THz - purple 


* Bond-breaking / ionizing
:: 'UV' is a very wide range (0.3um..10nm, 790 THz .. 30 PHz) that it is usually subdivided into UVA, UVB, UVC based on the varied effects.
::: UVA - 315–400nm, ~800THz,  (tanning beds. The little UVA from the sun that makes it to the surface causes at most minor skin damage)
::: UVB - 280–315nm, ~1000THz, (sunburns, skin cancer)
::: UVC - 100–280nm, ~1200THz (used for sterilization, ozone generation. Will burn flesh if strong enough, and damage your eyes. The [[ozone]] it generates from oxygen isn't ideal either)
:: Notes:
::: there are other classification, like far/middle/near UV
::: and there is [https://en.wikipedia.org/wiki/Extreme_ultraviolet extreme ultraviolet], 10-120nm
::: Because of the scale, the numbers are not very precise. UV could be said to start ''maybe'' around ~750THz, while e.g. 700 THz is still just visible purple
::: EM starts to be '''ionizing''' somewhere ''above'' UVA, so UVB and UVC are considered ionizing. 
::: Luckily, our atmosphere blocks almost all UVC, and a lot of UVB (and relatively little UVA) 
::: Blacklights often straddle visible and a bit of UVA, so are mostly harmless
::: you ''can'' get UVC germicidal sterilizing lamps, but you don't want to [https://www.youtube.com/watch?v=1m0TQjBRcFo]. You can usually tell by them having clear glass, because you need fairly special glass to even pass UVC. (No, blacklight/purple style UV does ''nothing'' in terms of germs)
::: window glass take out most UVB, though only some of UVA. This is part of why you won't easily get sunburned indoor - and your body produces less vitamin D if you never go outside.

* '''X-ray''' is roughly (10nm..10pm) 30 PHz to 30 EHz
:: Soft X-ray are those with lower energy (30PHz to 3EHz)
:: Hard X-ray (above ~5eV, 3EHz) have better penetration but do more damage

* '''gamma waves''' is above 30000000THz+ (30EHz), shorter than 10pm


<!--
More notes:

Radio

{{comment|(side note: the food cooker we call microwave is ''way'' more specific, namely 2.45 GHz, because that frequency heats water specifically)}}

: on names, TV, and radio: 
::: It's a generally very useful range, so a lot of other things use it too
:: radio in general had similar naming issues before, growing many extra names over time, which is why we ended up with HF, VHF, SHF, UHF, EHF (High, Very High, Super High, Ultra High, and Extremely High Frequency), and to the other side MF, LF, VLF, ULF, SLF, and ELF (Medium, Low, Very Low, Ultra Low, Super Low, Extremely Low
::: each associated with a few typical uses, for practical reasons 
:::: sometimes as simple as "size of the antenna" (roughly why Mhz to GHz are practical for domestic systems)
:::: directionality of typical antennas is also related
:::: and for reasons like maximum emission due to previously established rules - though this is more fluid
: for a general idea of radio frequency use {{comment|(the spectrum grew to very fragmented over time)}}
:: e.g. the very low frequencies are very low information, but still useful for things like navigation, time stations, and since it penetrates water better, submarine navigation
:: speech and audio is usually above ~30kHz, because below that the very limited bandwidth is impractical
:: ~100kHz is perhaps more useful for energy transfer (e.g. inductive heating, some wireless charging)
:: 
:: mobile phones are mostly around a few GHz, but are now pushing to a few dozen GHz for higher speeds at shorter range
:: ''lots'' of gaps are filled by mobile, aeronautical, amateur
:: 3GHz to 300GHz is a mix of many things. Things that are more unique (and somewhat reserved) include astronomy and satellite use
:: astronomy uses mostly radioteleslopes because of the 'radio window'[https://en.wikipedia.org/wiki/Radio_window], the fact that 15 MHz to 1 THz, is one of the two ranges that mostly just gets through our atmosphere (the other is visible light and some things next to it)
-->

==On EM and harm==
{{stub}}


'''For context'''

We are used to 'radiation' meaning 'nuclear' and 'danger'.

But any EM radiates. 
From the harmless visible light, to heat, to radios, to the harmful UV, gamma, and other nasty things in space.


We are used to 'photon' meaning 'visible light'.

But in physics the term is valid for ''all'' EM, from radio to IR to visible to gamma.




'''Harm how?'''

EM radiation can damage in one of two ways, and the distinction turns out to be a specific place, 
a specific threshold in the EM spectrum, which happens to lie in the UV range.


'''Below that threshold''' (what we call radio, thermal, optical), 
the only way to harm people is for there to be so much of it it heats them up too much.
This is easiest with a specific range of IR, but possible with ''any''.

Doing that enough to ''cook'' them is hard to do accidentally, and hard without them noticing (pain receptors being what they are).

Most natural sources aren't strong enough to bother us much - the sun's heat being the strongest example down here on earth's surface.

Most man-made sources of these are often not strong enough due to regulations, or if they are, they typically have fences around them.

But perhaps more practically, if the only mechanism is heating, 
it can't be damaging without first being noticeable (there are stories radio tower engineers having snacks melt in their pocket but not really noticing themselves), then painful, and only then being harmful. So this is not generally not an issue.


The only everyday example I can think of being potentially risky is a microwave - which is why those are shielded, and have [[interlocks]] based on its door.

Whereas e.g. your phone tops out at about 2 Watt (and it lowers it as much as the distance to the tower allows, to save battery),
and most variants have a [[duty cycle]] of order of 1:8 or so[http://www.techmind.org/gsm/], meaning the average power (which is what matters here) is an eight of that peak power.

Smush that between your face and hand hard enough that they recover all that power, half in your face and half in your hand, 
and that's still not an amount that matters. Your hand puts out more than that {{comment|(you put out ~100W ''overall'' when relatively idle, hands are ~1% of your body surface)}}. 



'''Above that threshold''', it turns out that it's no longer about collective effect - every individual arrival (and they are tiny and many) individually has enough energy to react with molecules.
: (The table above lists photon energy because it's around ~10eV that EM quanta are strong enough to be ionizing, that is, break molecular bonds - in the UV range)

This means that ''any'' amount of it can and will do damage to our cells.

We typically call this ionizing radiation, because that's ''how'' it does that damage to our cells (and to other things).




If you really wanted to heat people, you would generally choose the highest frequency you can easily generate,
because it more easily carries more energy.

Below ionizing radiation, the easiest happens to be the infrared range.

There is no accumulative cell damage until you use temperature that start cooking people,
and people will notice that.


<!--
That energy is photon energy * amount of photons, but it turns out we have a lot of regulations about the latter.

Most transmitters cover a large area so are a wide beam. Perhaps only a powerful narrow-beam point-to-point microwave transmitter might melt the chocolate in your pockets at any distance.
-->



Our bodies can deal with a tiny amount of ionizing damage - and continuously do, in the form of background radiation and the small amount of UV(-B) that isn't blocked by our atmosphere.
: and we still like to ''keep'' this low.



Note that there are other reasons for things to have more energy. For example
: Aurora are charged particles (which?{{verify}}) from the sun, which need to go moderate speed to even make it into our atmosphere{{verify}}.
:: But don't worry, it mostly happens 100-200 kilometers up, and only a small amount of it makes it to ground level, rarely enough to affect our infrastructure much {{verify}}

: [[cosmic rays]], which are actually ''particles'', have lots of energy because the ones that we detect are typically going near the speed of light, making their effective energy high enough to be damaging and, when these are also ''charged'' particles are then ionizing. 
:: But don't worry - they're rare enough individual events, so they bother computers more than us.


<!--
* the sun puts out pretty much a IR, optical, and UV
: search for things like {{search|sunlight spectrum}}
-->

==How much gets to us from space?==

Broadly, see this image from NASA, via wikipedia:

[[File:Atmospheric electromagnetic opacity.svg.png|900px]]


==Units of ME (and other) radiation==

<!--

Magnetic field (H field)
: Tesla: magnetic flux density 
: the older Gauss (from CGS) is 0.0001T


Electric field (E field)
: V/m
: 
1 V/m refers to a potential difference of 1 V between two points 1 m apart.

Volt per meter is the standard unit of electric field (E field) strength


Ionizing
: Sievert 
 


-->

==Where are everyday devices on the EM spectrum?==

<!--
Note: '''A lot of these figures are rounded''' for a rough idea


Inductive cooking: usually around 24kHz

[[Wireless charging]]: 100-200kHz (WPC), ~300kHz (PMA), or ~6.5GHz (A4WP)

RC cars/planes - classically often 27MHz, can also be 49MHz, 50MHz (HAM licensed), 72MHz (US?), 75MHz (US?), 433 MHz, 869 MHz, and now 2.4GHz

FM radio: roughly 85MHz through 105MHz

Marine and amateur radio: various bands between 5MHz and 250MHz (and more)

TV: classically mostly between 50MHz through 250 MHz

Short range remote control - often 315MHz, 426MHz, 433MHz, 868MHz, or 915MHz, depending on where you are ([[License-free sub-gigahertz]] bands)

WiFi: 2.4GHz or 5GHz  (at ~0.1W to ''at most'' 1W)

Cell phone: 0.85GHz, 0.9GHz, 1.8GHz, 1.9GHz, and/or ~6GHz, and a few higher ones with 5G not really in use yet
: 0.1 W of power. A cell phone emits about 1 W

Microwave oven: 2.45GHz (at hundreds off Watts, which hundreds times higher energy than most other things around here)

Lighting: ~600THz

Blacklights: varies a litte. Some things we call UV are more of a deep purple and not really UV-A proper, around 750THz / 400nm{{verify}}, but enough to luminesce with certain materials. {{verify}}

Damaging stuff: >800THz

Sunlight 
: in space: most frequencies, to different degrees  (it acts like a [https://en.wikipedia.org/wiki/Black_body black-body source])
: at earth's surface, mostly: 
:: the 'radio window'[https://en.wikipedia.org/wiki/Radio_window], 15 MHz to 1 THz
:: optical range (430&nbsp;THz to maybe 790&nbsp;THz)

 
: Aside from that peak, our atmosphere also happens to filter out a bunch of frequencies. There are two ranges that make it through mostly unimpeded:
:: visible-light[https://en.wikipedia.org/w/index.php?title=Visible_light]
:: the 
What makes it to earth's surface is largely optical, and a bunch of radio, the rest is filtered to moderate or high degrees


Medical X-ray: Varies, but it's usually higher in the PetaHertz-to-ExaHertz range {{verify}}

Radioactivity (background)

Radioactivity (deadly)


-->
===Variations in IR===

===Variations in UV===

===Variations in X-ray===

===Variations in full-body scanners===
<!--

Full-body scanners exist in three major variants:

* Millimeter wave backscatter scanner - in the EHF radio band, roughly 24-30 GHz, ''well'' under ionizing and visible range

* Backscatter X-ray - X-rays soft enough to only really measure reflection, meant to make it through clothing but ''not'' penetrate skin
:: it ''is'' ionizing, but apparently less severe than even the flight itself ({{verify}} dose)
:: 0.02 .. 0.1 micro-Sievert per screening{{verify}}

* Through-body X-Ray - classical X-ray that goes ''through'' people
:: rare to see as full body scanners{{verify}}
:: 0.1 to 5 micro-Sievert per screening   (still not very high because it's near-instantaneous and doesn't need very detailed contrast)


Apparently there's also infrared scanners, that are passive and do little more than point out there may be an object under someone's clothing

-->


==Infrared notes==

{{stub}}


While infrared often refers to near-infrared, it's actually a fairly huge range, split into ranges in a few different ways[http://en.wikipedia.org/wiki/Infrared#Different_regions_in_the_infrared]. 

One common seems to be the near/mid/far split {{comment|(near/far relative to the visible spectrum, 380nm..740nm)}}, like:

* Near-Infrared
** Wavelength approx 740nm to approx  2500nm
** IR LEDs are usually near-IR, typically somewhere within 700...1000nm
** most relevant to optical astronomy{{verify}}, though 


* Mid-Infrared
** 2500–25000nm

* Far-Infrared
** 25000–1000000nm{{verify}}, getting close to microwave region


See also:
* http://www.ipac.caltech.edu/outreach/Edu/Regions/irregions.html
* http://en.wikipedia.org/wiki/Infrared

<!--

What emits each?
Lists you'll find may be focused on 

https://www.icc.dur.ac.uk/~tt/Lectures/Galaxies/Images/Infrared/Regions/irregions.html




'''Near infrared''' from thermal emission comes from very hot objects - 700..5000K, the upper range of which is suns and stars.
And those are such broad emission that it is likely also visible.

We can also make very narrow-band near-infrared light, e.g. with LEDs.


'''Mid infrared''' from thermal emission is about objects at less ridiculous ranges, including both human-comfort temperatures and actually quite a bit colder and hotter - maybe 100 to 700K.


'''Far infrared''' from thermal emission is very cold things. In astronomy, it may point out that there's a lot of matter in a place, which might eventually become a star but right now wouldn't show in mid or near IR. 



The hotter it is, the further it comes down through IR closer to visible. This intuition is muddied a little by that some things that are that hot are will have a ''very broad'' emission spectrum so hard to distinguish, and possibly be emitting other light for other reasons (e.g. from being on fire, or ''being'' fire).



Near infrared isn't necessarily that much more useful than optical, though the instruments are very similar (possibly just one sensor swap away),
and the fact that e.g. certain dust clouds are more transparent in infrared than in optical is certainly useful.

Warmish things emit most in mid-infrared{{verify}}, so mid-infrared is useful to find things like asteroids.

Far infrared mostly shows cold things, in astronomy revealing gas and dust clouds



Trees light up in IR because chlorophyll absorbs red and blue. This leaves green (in the middle of the visible spectrum) and IR just outside it (and a lot of others we're not talking about now), for us to see/detect. Infrared photography is mostly sensitive to near-infrared to start with, so .




-->


===Infrared and cameras===

See [[Photography_notes#Infrared]]

===PIR===

<!--

Passive Infrared (PIR) sensors are typically used for motion detectors,
which themselves are usually recognizable by plastic facets - a fresnel lens to determine the area to be sensitive to (which may also be a filter).


The sensor is a [https://en.wikipedia.org/wiki/Pyroelectricity pyroelectric sensor],
typically sensitive around around 10um, somewhat mid-infrared, correlated to temperature.
You can see them as a one-pixel sensor that reacts to incoming heat, 
typically used in a way where they trigger from a fast-enough ''difference'' in temperature.
(which is why they can also trigger by drafts, coolants, etc.)

There are some variations like using two of them for differential detection,
which can be more robust, avoiding some false negatives.


Passive just means it doesn't send anything out. 
The heat from people and such means it won't detect further than a handful of meters.

The range they are sensitive doesn't pass through window glass much, because they often do their best to not pass temperature too easily.

-->

===Communication===

Two-directional communication is typically half-duplex because a device can easily be blinded or confused by its own signal.


'''Consumer IR''' (TV remotes and such)
* Often uses a intermittent patterns of pulses, not continuous sending
** Pulsing with low duty cycle also means you can pulse the LEDs with more current without destroying them.
** sending and a specific speed of pulsing is something that won't occur naturally, so helps confusion from environment IR. 
*** Carrier usually 38kHz. More generally it's somewhere in 33..40kHz or 50..60kHz, often 38kHz, 40kHz, or 36kHz
* In the case of remotes there are hundreds of variant protocols (that is, bit patterns that are specific to brands and devices)
** Universal remotes usually have a lookup table from brand-and-model to one of hundreds specific code sets that the remote supports
** and occasionally the ability to learn codes from an example

http://en.wikipedia.org/wiki/Consumer_IR



'''IrDA'''
* Speed: 2.4 kbit/s to 1 Gbit/s (faster speeds primarily at close range)
* Modulation: baseband, no carrier
* Has a few different layers 

http://en.wikipedia.org/wiki/Infrared_Data_Association

===Heating===

<!--
"Is all infrared heating?"

No.


But things that are heating tend to be so wide infrared. More specifically, anything that is warm (warm radiator sort of temperatures) to glowing hot - see [[incandescence]] - tend to omit much of their energy in a lot of the infrared part of the spectrum (the IR range is quite large)- and depending on what it is exactly can be anywhere in the large chunk of the [[EM spectrum]] that we call infrared (approx 1mm / 0.3THz through approx 0.75um / 400THz).

And yet it is still primarily the far part of that (the 0.3THz / 1mm end, Far InfraRed (FIR) and IR-C) that we associate with heat. 


Why?

IR-A is a little easier to emit (though in practice often means a bright light - e.g. halogen is more on this end{{verify}}), 
but the FIR end is absorbed more easily.



(Also keep in mind that enough of ''any'' EM will heat things and people)



That said, Infrared LEDs are both low current ''and'' on the IR-A end, 
so 









Say, the sun. Or a big chonky wire/resistor 
that gets hot because we put a couple hundreds or thousands of watts of power through it.


I've seen adds for special FIR heaters. It's natural, like the sun! It's more efficient!


"Natural!" 

So is gas, wood fire, and most other sources of heat.

And I'm assuming you mean the resulting radiation - as components they are chunky resistors that get hot.




"More efficient!" 

Yes and no.


Whenever you generate heat for the sole purpose of heat, and inefficiency is... also heat.
If the heater is in the same place, the efficiency of what you're doing is pretty near 100%.


Ir tends to refer to electric infrared heaters.

In most places electricity is more expensive per Watt than e.g. gas.

That said, heating with gas tends to mean convection. 
When you want to heating all the air in the room, then gas can be more efficient.
In terms of money anyway, at least until the 'non' in 'nonrenewable' turns out to be an issue (electricity was always an transfer method, not a source).


Radiative heating can be more focused. 

This can be both good and bad. 

Say, if you need to keep some things in a warehouse warm enough to keep them well, and you don't want to build a little room,
Compared to heating all the air in the warehouse, and insulating for that to not be pricy, 
some focused IR radiation may be a great solution.
: ...while the the rest of your warehouse is probably still damp and moldy


If you're heating an office, you might find that the side of you facing the heater is very comfortable
: ...while the other side is freezing.


If you want a terrace heaters on your porch, or outside your bar, that would make zero sense to do with convection because most would blow away before it heats anyone, while air is mostly transparent to IR so just hits what you point it at. 
Whenever that's not 100% people that's still a a waste of energy, but if the customer draw is worth it, your choice.

On your porch, it's pretty decadent in that .



'''Especially good for us'''

-->

=Other EM-wavey notes=

==Waveguide==

<!--
A waveguide guides waves in one direction, meaning minimal loss.

Which are probably the two largest reasons right there.

Reasons to use it include
: pointing it on one direction, like 
: having less loss than the [[inverse-square]] of omnidirectional transmission



Waveguides are often either rectangular or circular.



'''EM waveguides'''

Electric
Magnetic

TE - Transverse Electric

TM - Transverse Magnetic


https://www.youtube.com/watch?v=iLngL2Zj2UI

https://www.youtube.com/watch?v=r9-m17IPOco

https://www.youtube.com/watch?v=bwreHReBH2A



'''Audio waveguides'''

Waveguide speakers

don't have a flat frequency response, so crossover is hard to design, and if you do it wrong they sound worse.

Done right, they give a larger-area sweet spot{{verify}} so a more stable stereo image (than omnidirectional), for more people in the room.



So, tweeter horns are waveguides of sorts


But because of the low frequencies involved, 
a flat baffle speaker basically qualifies to some degree as well. Just because it's directional.





https://www.youtube.com/watch?v=w2iTCm0xpDc
-->


==Multipath, diversity, beamforming, beamsteering, MIMO==


<!--
[https://en.wikipedia.org/wiki/Multipath_propagation Multipath refers] to receiving multiple copies of a signal that took different paths.

If this happens out of your control, e.g. due to refraction and reflection in general (in longer range communication mostly from the atmosphere), it implies constructive and destructive interference, and phase shifting, which can also amount to locally lower-than-intended signal strength.


In GPS, multipath is one reason the location can seem to jump around more when you're in canyons or between highrises - but still reasonable, because this mostly happens in the last piece of the path.

In radar, it can causes ghost copies of real objects

In audio, multipath is related to echo and reverb (room scale stuff, mostly because audio in longer distances attentuates too much).

In analog signal reception, you may sometimes get a signal blur over a little time (sometimes even a singular copy due to a mountain to the side, or such), that will also and things that cause things like analog TV's ghosting.



There are various ways of using multiple antennae to help with these issues and others.


In shorter range, '''[https://en.wikipedia.org/wiki/Antenna_diversity diversity]''' refers to the fact that one of your antennae may receive a specific better signal than another, and a system aware of this (e.g. able to select antenna) will be able to receive that signal better.

Similarly, systems may be able to figure out transmit diversity as well, e.g. by assuming the way they receive from a non-moving station is also a good indication of how to send back to it.







On the other hand, things like MIMO try to take advantage of multipath propagation.


An [https://en.wikipedia.org/wiki/Antenna_array array antenna] / [https://en.wikipedia.org/wiki/Phased_array phased array antenna], multiple transmission antennas coherently transmitting the same signal slightly out of phase gives constructive interference in specific directions.

This is one form of '''[https://en.wikipedia.org/wiki/Beamforming beamforming]'''.

'''[https://en.wikipedia.org/wiki/Beam_steering beamsteering]''' when there is on-the-fly control of this direction (timing/phase difference between the antennas.


Similarly, multiple receive antennas receiving the same signal can, with a little phase analysis, let you estimate roughly what direction a signal came from.


MIMO has become a fuzzy term. It can refer to anything using multiple input, multiple output antennas, 
or both, which are all different techniques.

Still, MIMO often refers to smart antenna use that give more bandwidth, 
''other'' than beamforming and diversity. 


This comes in part from WiFi (also mobile), which introduced a few techniques roughly at once (multipath, MIMO, antenna diversity, orthogonal frequency division multiplexing), and the average person picked it up as being called MIMO.




For example, {{search|mmWave sensors}} (roughly, everyday-object radar) can be run in a few modes.

The ones that have multiple tx and rx antennas can e.g. be run in transmit-one-receive-all, 
while making all tx transmit in unison (=beamforming) puts most of the constructive interference directly in front, meaning you get stronger / longer range detection in the middle ~22 degrees, at the cost of worse detection away from there.



https://en.wikipedia.org/wiki/MIMO

-->

==RADAR==

<!--

===CW radar===

https://en.wikipedia.org/wiki/Continuous-wave_radar


====FMCW radar====
Frequency-Modulated Continuous-wave radar



https://en.wikipedia.org/wiki/Continuous-wave_radar#Modulated_continuous-wave



===CFAR===

CFAR is a filter that requires a peak to be some amount stronger than its directly adjacent signal.

It is effectively a system that automatically follows the noise that will vary with with position, time, etc.


It comes from radar, where it was circuitry that decided a reasonable threshold, the power level above which any return is probably from a real target.


====Why this way?====

Say you want to detect events in a noisy signal.

When you can assume the background noise is constant with time and space,
the probability of detection is largely a function of signal-to-noise ratio of the target return,
and the probability of ''false'' alarm comes mostly from the [[probability density function]] of that noise (itself often Gaussian), and would be fairly constant.

Yet in real systems, the background noise changes both over time and space (clutter and interference), so a changing threshold is necessary to maintain a constant rate of false alarm.

(False alarms are themselves accepted as a necessary tradeoff with false negatives)


CFAR's particulars come from what was easy to implement with analog circuitry at the time, but was clever enough that its modern implementation basically still imitates it fairly directly.


====Variants====

Implementation details vary between:
* CA (Cell-Averaging)
: uses surrounging noise average: averages a number of cells around the cell under test
: often avoiding the directly adjacent one or two cells (guard cells)
: (sort of a kernel weighing adaptive-threshold thing{{verify}})

* CAGO / GO (Cell-Averaging Greatest-Of)
: deals slightly better with adjacent, non-homogonous clutter

* CASO / SO (Smallest Of)


* OS (Ordered Statistic / Ordered Sort)

* OS combined with GO / SO
:: CAOS (Cell-Averaging, Ordered Statistic{{verify}})
:: OSGO
:: OSSO

* CASH (Cell Averaging Statistic Hofele)
** 

* MAMIS (MAximum MInimum Statistic)
: basically CASH in a specific hardware implementation?

Also, for some applications it makes sense to do it in log, or in linear magnitudes.



https://www.radartutorial.eu/01.basics/False%20Alarm%20Rate.en.html

http://www.jcomputers.us/vol6/jcp0606-29.pdf

{{search|Object Detection with Automotive Radar Sensors using CFAR Algorithms}}


-->




[[Category:High school stuff since forgotten]]
[[Category:Nerding around]]