Difference between revisions of "Some physics related to everyday life"

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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.
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)}}
(The main names here seem to be [[BPA]], some [[phthalates]], and some others)
====Metal (mostly)====
====Metal (mostly)====

Revision as of 17:44, 23 June 2021

EM spectrum

Quick(ish) reference

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

There are a lot of images you can find that are much nicer than this.

(I was doing an experiment which I have not nearly finished yet)

A broad table (from wikipedia) for some overview:

Light comparison
Name Wavelength Frequency (Hz) Photon energy (eV)
radio 1 meter – 100,000 km 300 MHz – 3 Hz 1.24 μeV – 12.4 feV
Microwave 1 mm – 1 meter 300 GHz – 300 MHz 1.24 meV – 1.24 μeV
Infrared 700 nm – 1 mm 430 THz – 300 GHz 1.7 eV – 1.24 meV
Visible 400 nm–700 nm 790 THz – 430 THz 3.3 eV – 1.7 eV
Ultraviolet 10 nm – 400 nm 30 PHz – 790 THz 124 eV – 3.3 eV
X-ray 0.01 nm – 10 nm 30 EHz – 30 PHz 124 keV – 124 eV 
Gamma ray less than 0.01 nm more than 30 EHz more than 124 keV

The photon energy becomes important to life, because above approx ~10eV (in the UV range) EM quanta are strong enough to break molecular bonds -- see ionizing radiation.


  • Below 3kHz or so
static field
  • 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). Microwave came in as a "well, turns waves this small are also usable for radio-like uses" (they were just harder to generate at first) (side note: the food cooker we call microwave is way more specific, namely specifically 2.45 GHz, because that frequency works well to heat water specifically - though other bands also heat)
...similar how to over time, we grew names like
HF, VHF, SHF, UHF, EHF (High, Very High, Super High, Ultra High, and Extremely High Frequency) radio waves
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", and also legal reasons
a general idea of frequency use (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
AM music radio typically within 500MHz..1500MHz (60cm .. 20cm)
TV and FM radio typically within 50MHz .. 1GHz (6m .. 30cm)
mobile phones are mostly in a few places between the ~1GHz to 40GHz range
and lots of gaps are filled by mobile, aeronautical, amateur
3GHz to 300GHz has such a mix of uses but more on the astronomy, satellite sort
antenna size is largely related to whe wavelength, which is part of why MHz to GHz were practical, also for domestic use
directionality of typical antennas is also part of typical use of bands
Radar is more a technique, and there are different Radar-y use between 3Hz and 100GHz[1]

  • the terahertz gap, roughly 0.3cm .. 30um, 0.1 THz to maybe 10 THz
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's more than one sub-categorisation, but we frequently go by roughly
FIR (far infrared) (1mm..14um, ~0.3THz..20THz)
IR-C (14um..3um, ~20..100 THz)
IR-B (3um..1.4um, ~100..200THz),
IR-A (1.4um..0.75um, ~200..400THz)
Infrared is called thermal, but it's only perhaps half of the sun's thermal delivery, and only part of IR is particularly warm

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

  • Bond-breaking / ionizing
'UV' is a range so very wide (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, from sun causes relatively 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)
there are other classification, like far/middle/near UV
and there is extreme ultraviolet, 10-120nm
Because of the scale, the numbes are pretty 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
Our atmosphere blocks almost all UVC, 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 [2]. You can usually tell by them having clear glass, because you need special glass to even pass UVC. So yes, the regular purple UV does nothing in terms of germs.
  • X-ray is roughly (10nm..10pm) 30 PHz to 30 EHz
Hard X-ray (above ~5eV, 3EHz) have better penetration but do more damage
Soft X-ray are those with lower energy (30PHz to 3EHz)
  • gamma waves is above 30000000THz+ (30EHz), shorter than 10pm

Everyday device and EM

Full-body scanners

Practical questions

Around the microwave oven

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

A microwave oven roughly consists of a microwave source (a magnetron), a microwave-insulated chamber, and a way to guide waves from just into there (a waveguide).

Microwaves deliver energy most efficiently into polarized molecules. It vibrates them (at 2.45GHz (wavelength is ~12cm)), creating friction and thereby heat.

There are various molecules that react, but the most abundant one in that list is water (though also fat and sugar(verify)).

By implication, any food food with high enough moisture content can be heated in a microwave.

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

Microwaves penetrate up to about 2-3cm (~1 inch) of dense-ish food (verify).

This is one reason you may see unevenly warmed food. If heating something large and round, the rest would have to be cooked via conduction, which can take a lot longer, which you can do but at this point an oven will probably be just as effective.

Another reason for temperature differences is the possibility of standing waves, which amount to some physical positions getting less energy than others, which is largely solved by the rotating platter (but still it can be that e.g. the center of pizzas may be cold because they don't move enough(verify)).

Bad ideas

Some plastics

Whether a plastic is microwave safe is not about direct heating, and more about what they might release when hot enough.

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. (The main names here seem to be BPA, some phthalates, and some others)

Metal (mostly)

A few foods

A few ceramics

Other potential problems

Sudden boiling liquids

It is possible for some liquids to [superheat], which for these purposes means their temperature is above their boiling point but do not look like they are boiling.

This doesn't easily happen.

It's roughly because there is nothing to start the movement we call boiling. This only really happens when the liquid can heat very uniformly.

The movement involved in picking it up is usually enough to start the boiling, as does adding something at a different temperature (spoon, another ingredient), as do even small amounts of various impurities (which is why this isn't so easy to do with tap water).

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


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