Wikipedia:Reference desk/Archives/Science/2021 December 11

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December 11

Physics: which materials reflect light the most?

If you have a room where you replaced material that reflect (absorb) light from 99%, to 98%, then your room is 1% brighter. What materials reflect light the most / absorb the least? And what is this property called? 67.165.185.178 (talk) 11:04, 11 December 2021 (UTC).

See reflectance. Mikenorton (talk) 11:47, 11 December 2021 (UTC)

If the reflectance goes down from 0.99 to 0.98, the room becomes less bright. Shiny metals, such as used for the reflective layer of mirrors, have a vert high reflectance. A layer of deposited soot has very low reflectance. For the "blackest black" yet developed, see here. See also the runner-up, Vantablack.  --Lambiam 17:25, 11 December 2021 (UTC)

Weird, metals are conductors, they allow for electron to flow. Insulators block electron flow. I would expect metals to absorb electrons more, but it seems the opposite is true, metals reflect photons the most. 67.165.185.178 (talk) 19:18, 11 December 2021 (UTC).

Photons and electrons are very different kinds of particles. Photons can be absorbed by donating their energy to an electron, allowing it to jump to a (higher energy) excited state. What makes a metal electrically conduct is the presence of free electrons, which can occupy a continuous spectrum of energies without jumping between states. That is also why they can collectively respond to the incoming electromagnetic wave to form a counter wave that materializes as an emitted photon – just as if the incoming photon bounces back. Or is the emitted photon another photon?... in the quantum world, the distinction between it being the same or a newly created photon loses its meaning.  --Lambiam 22:16, 11 December 2021 (UTC)

I'm wondering how the fuck anything is transparent. If electron clouds block, reflect, absorb and scatter visible light so much and make the universe opaque before 380,000 years old then how is everything not opaque with the exception of things thin enough that some rays do not have to penetrate many electron clouds. Are the electron clouds transparent if you are not trying to penetrat with a spectral line? Then why can't I see the broad-spectrum sunlight through a graphite brick? Why do some substances block a broadband part of the spectrum seemingly for no reason but transmit nearby ones well? Why is crystal glass colorless when it has such a high percentage of orange lead oxide? Sagittarian Milky Way (talk) 21:04, 11 December 2021 (UTC)

See Transparency and translucency - transparent materials such as glass lack internal defects, like grain boundaries. Mikenorton (talk) 21:16, 11 December 2021 (UTC)

So the magic electrons = transparency obliterating thing of early universe must not be some inherent "photon bothering property" of electrons then (presumably enhanced by Heisenberg or something but maybe space clearing up didn't have much to do with that). The ability to see through quadrillions of fiber optic atoms in a row had been confusing me for well over a decade. Why is hematite opaque? It has an index of refraction higher than diamonds and silicon carbide gems but no one can make a crystal flawless enough. Sagittarian Milky Way (talk) 01:25, 12 December 2021 (UTC)

And I've found an answer: Refractive_index#Microscopic_explanation. Sagittarian Milky Way (talk) 22:47, 12 December 2021 (UTC)

Doesn't make it any clearer.  Card Zero  (talk) 00:22, 13 December 2021 (UTC)

I remembered that clouds and especially snow are big reflectors of IR, forgot what are big reflectors of UV, but that does not necessarily mean they are good reflectors of regular light? 67.165.185.178 (talk) 14:28, 12 December 2021 (UTC).

According to the Internet snow and cloud reflects near IR but the curve quickly goes near zero and has absorption troughs. Maybe they're from the photons jiggling the chemical bonds come to think of it. According to reflectance aluminum reflects UV so well that it actually gets better without stopping from red through running off the 200nm edge of the chart, and according to fused quartz pure quartz glass transmits to lower wavelengths than regular glass, but at some point UV becomes harder to work with, as the electronics companies trying to switch to extreme ultraviolet lithography have found out. Getting shorter than UV it starts to penetrate more and more things and becomes hard to focus and space telescopes like EGRET switch to reflecting the rays at very glancing angles or having deep holes pointing slightly different directions from their neighbors with simple ray counters at the bottom causing bad resolution (unless they've launched a better one since I've last followed them) Aluminum is an awesome mirror material, it's the 2nd cheapest metal by volume and sometimes even by weight, 2nd most common metal in the crust, easy to work, easy to "paint" on glass in thicknesses from easily wipeable to more robust, rust stops before harming vis-mid IR(+?) optical properties much, not very week or easy to melt (though easy to scratch), the lightest common metal (density is glassoid), non-magnetic, not very harmful to humans (70kg non-dialysis adults can eat up to 0.1 ounces per day), recyclable and reflects at least ~86% and up to ~99% at all wavelengths from "so small air is opaque" to over 20 microns which is so long that the mirror would glow like a light bulb and mess up your images unless the mirror was cooled to temperatures that would cause a lot of the atmosphere to rain into a layer several meters deep if they were worldwide). Shopping for a waterproof object that transmits the same wavelength range well would be a pain in the butthole (even just one wavelength at a time) Fluorite#Optics is transparent from 0.15 to 9 microns and is expensive. Sagittarian Milky Way (talk) 17:34, 12 December 2021 (UTC)

Temperature of black and white cars in winter.

So we know that black color cars absorb heat the most, white the least. In the summer times, the temperature inside a black car could be 10 to 17 F more hotter than inside a white car. But what I'm trying to find is, winter examples. A Google search of "temperature of black and white cars winter" still shows nothing but summer results. Even adding the word "cold" before winter, still shows summertime results. However, I have a feeling, that the disparity, is going to be less than summertime results? Making a black car in the cold winter, no real applications for warmer temperatures? 67.165.185.178 (talk) 12:44, 11 December 2021 (UTC).

Heat is going into the car at one rate, and going out of the car at another rate, and there's a bunch of heat equations for dealing with different situations, which might fill an engineer with joy. Point is, anyway, the car is not only getting more radiation in sunny weather, it's also heating up because the heat can't leave very fast, because the outside air is also warm. If the outside air is extremely cold, the rate of heat loss (assuming it's at least a bit warm inside) is much higher than the rate of heat gained from the thermal radiation of the sun - even if it's sunny and cold at the same time. That article has a nice graph from NASA (twice over!) showing that white paint actually absorbs heat radiation at an unexpectedly high percentage (but a mirror doesn't).

For every situation (a white car on a sunny day in the arctic, a black car on a cloudy day in the tropics) there will be a thermodynamic equilibrium, the temperature inside the car the system eventually settles on. I think that's the right term, anyway. I can't actually read the article to find out because I don't have all day to figure out what it really says, so I'm not even sure what the point of linking you to it is. I hate the inaccessibility of mathematics.

Edit: I found a calculator for temperature of a car in sunlight at different outside temperatures, over time. The foot of the page has "The color of a car does not have a significant impact ...".  Card Zero  (talk) 15:08, 11 December 2021 (UTC)

There is something incongruent about that calculator. I enter in temperatures of below freezing, and it still shows getting marginally warmer over time. But there should be a point where the longer you leave your car outside, the colder it gets inside. 67.165.185.178 (talk) 17:23, 11 December 2021 (UTC).

I thought that as well, but deleted my objection, because the calculator starts with the inside of the car at the same temperature as outside, then of course it increases under sunlight like the temperature in any greenhouse, and (presumably) levels off at a temperature which is warmer inside than outside, but still potentially horribly cold.  Card Zero  (talk) 17:31, 11 December 2021 (UTC)

The color of the interior of a given car might be more important than what color the body is painted. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:40, 11 December 2021 (UTC)

Four heat transfer processes can be distinguished (if the car windows are closed and the AC is not running), each of which involves predominantly one type of transfer. (1) Heat captured from incoming radiation. (2) Heat exchanged by convection between the outside surface of the car and the outside air. (3) Heat exchanged by conduction between the outside and inside surfaces of the car body. (4) Heat exchanged by convection between the inside surface of the car body and the interior air. The car colour is only important for (1). In equilibrium, process (4) stops exchanging heat: the temperature of the interior air has become the same as that of the inside surface of the car body. This simplifies the equations. The radiation emitted by a hot car is not entirely negligible, but of minor importance compared to the heat loss by convection.  --Lambiam 17:07, 11 December 2021 (UTC)

Okay, a separate question, and this can be applied to clothing/coats. After black and white, the next colors are the Roy G. Biv spectrum. So wearing red is the best color to remain cool in the hot summer, and violet to remain warm in the cold winter. But what if you were to compare dark red, with light violet? Would that make it the same temperature effect? Gray is not a color in the spectrum, but I presume wearing gray is like the halfway point from wearing black vs. white, I wonder if wearing dark red and light violet is the same point? 67.165.185.178 (talk) 17:27, 11 December 2021 (UTC).

In cold weather, the effects of clothing colour will be orders of magnitude less that the effects of the clothing's material, thickness, and insulation (within and between layers) that shed water from precipitation, prevent external air penetration, and trap and retain body heat.

In hot (and presumably sunny) weather the most important colour effect would be reflectance of infra-red light. A garment red to the eye might still absorb greatly in infra-red; conversely a garment violet to the eye might also reflect infra-red well: the reflectance spectra of materials is not necessarily the same shape as an ideal black-body radiation curve. {The poster formerly known as 87.81.230.195} 90.205.227.133 (talk) 12:03, 12 December 2021 (UTC)

Objects at around room temperature emit radiation in the infrared band, but the sun is not such an object, so maybe visible light energy is significant when it comes to staying warm/cool under sunlight? "In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared ... 42 to 43 percent visible". Then, from Infrared#Heat, "Infrared light from the Sun accounts for 49% of the heating of Earth, with the rest being caused by visible light that is absorbed then re-radiated at longer wavelengths."  Card Zero  (talk) 13:51, 12 December 2021 (UTC)

You say the sun is no such IR object, but then say it emits 52-55% infrared? 67.165.185.178 (talk) 14:15, 12 December 2021 (UTC).

I mean they emit all, or nearly all, or most, of their energy in IR. Except, say, in a steelworks, where objects are hot enough to glow brightly. Even so, IR probably still accounts for most of the energy. It's just that with the sun, it seems that the visible component of the energy starts to be a significant proportion.  Card Zero  (talk) 14:39, 12 December 2021 (UTC)

So there is no UV-light affect for colors of clothing? Compared to IR? (In terms of temperature?). And in winters, there is still a difference between cloudy vs. sunny-days, right? 67.165.185.178 (talk) 13:54, 12 December 2021 (UTC).

Sunlight says only 3% to 4% of the energy reaching a person on Earth is UV. However, if your clothing is a pure spectral color, that means it's absorbing only a very narrow band (which is why it will look bright), and might be absorbing an even lower percentage of the available energy. (By the way, don't confuse UV with visible violet, if you were about to.) What you want is to absorb as wide a band of the spectrum as possible: so if the resulting clothing color isn't black, it will be dark, and probably some kind of brown - but perception can be deceptive. Pointillism mixes colors using adjacent bright dots, meaning that "bright brown" is possible. (Come to think of it, so does any computer screen, so you can experiment in a good graphics editor: I think the brown turns into purple or orange usually, so perhaps darkness is part of our definition of brown.) Best just to pay attention to brightness/darkness as an indicator of reflected/absorbed (visible) energy.  Card Zero  (talk) 14:30, 12 December 2021 (UTC)

" so perhaps darkness is part of our definition of brown" Yep. One common phrasing is that "brown is dark orange" (see our own article on brown). --Khajidha (talk) 15:01, 12 December 2021 (UTC)

Okay, back to clothing again, I did a Google "what colors absorb IR the most" and got into a clutter. Some do not necessarily say the Roy G Biv, but they all agree about black and white as the most opposites. Then, I found this on a physics stack exchange comment from a physics PhD: "People wearing dark clothing then wasn't an accidental cultural thing. Heat does not just come onto us from the sun. It also comes off our warm bodies. When all that body heat hits the white clothing we might be wearing, it also gets reflected right back into our bodies. So when we wear white, we are actually heating our bodies. This means that the best color to keep cool, is black. Black does absorb all wavelengths coming from the sun, but black also absorbs energy leaving our bodies, instead of reflecting it back into our bodies."

So, intuitively, this is the opposite, but, if you combine these 2, then, the best way to keep warm, from wearing 2 layers of clothing, would be, wear white on the inside, and black on the outer layer. 67.165.185.178 (talk) 15:05, 12 December 2021 (UTC).

But, if asking what's the best color to wear in the summer weather to keep cool, you got contradicting factors. If you're only in the shade, black is the coolest, but if you're under sunlight, then whether wearing white or black, you got contradicting factors. 67.165.185.178 (talk) 15:37, 12 December 2021 (UTC).

a study of loose-fitting robes worn in a desert concluded that it makes no difference, because in that situation, airflow is what matters. It's a much debated question. There should be other real-life experiments out there somewhere.

Here's some guy from Wired experimenting with T-shirts and an infrared camera, mainly confirming that white clothing doesn't reflect significantly more infrared than black.

Here's somebody from Bustle trying out wearing black all summer. Subjective, but she reports not feeling any hotter than on other years.  Card Zero  (talk) 20:06, 12 December 2021 (UTC)

Yes, or even better, a space blanket instead of the white layer - except I guess there's the problem of what happens at the boundary between the two layers. Maybe the black layer will do nothing except heat itself up, because of the reflective effect of the other layer.  Card Zero  (talk) 16:09, 12 December 2021 (UTC)

Black and white tell about reflection in visible light, which has nothing really to do with reflection in infrared. For outgoing radiation, reflection in infrared is what matters. But most heat leaves the body+clothes by conduction, convection and evaporation, not by radiation. Only when the other factors already get minimised, minimising radiation losses gets interesting.

That doesn't mean that in a hot, sunny desert wearing white is always better than wearing black. Your body is colder than the environment, so the heat flow through your clothes other than by evaporation goes inbound and it's best to minimise this flow. White clothes transmit a large fraction of the light, so it can reach your skin directly and heat it (along with the UV radiation doing damage). Black clothes absorb the radiation, so that the outer surface gets hotter, but this is somewhat insulated from the skin. Cooling to the body is then provided by air flowing underneath the clothes, where sweat can evaporate. PiusImpavidus (talk) 09:55, 13 December 2021 (UTC)

• Then, I found this on a physics stack exchange comment from a physics PhD: "(...) When all that body heat hits the white clothing we might be wearing, it also gets reflected right back into our bodies. So when we wear white, we are actually heating our bodies. This means that the best color to keep cool, is black. Black does absorb all wavelengths coming from the sun, but black also absorbs energy leaving our bodies, instead of reflecting it back into our bodies."

Well, I have a physics PhD as well, but my thesis probably contains more heat equations than that of the person who wrote this, because it is just incorrect in multiple ways.

1. As explained above, at ambient temperatures on Earth, radiation mostly occurs in far infrared (around 10 μm), so that "black" and "white" clothes actually have quite similar spectral properties at the wavelengths that matter. All the difference between reflecting or absorbing body radiation is therefore independent of the visible-light color of the clothes.
2. More importantly, and as was hinted to above, in the exchange of heat between one’s clothes and one’s body, radiation plays a small part - most of it is due to conduction or convection. If you wear stockings, you will feel that your legs are warmer with them than without, even though they can be transparent (well, in visible light, but I bet they are IR-transparent as well).

Tigraan^{Click here for my talk page ("private" contact)} 10:49, 13 December 2021 (UTC)

So, I think what PiusImpavidus and Tigraan is saying is, clothing in summer is more dependent on whether temperature is above or below body temp? ~98.6 F? Above body temp, find the clothes least-heated by sun (white), below body temp, find clothes least heated by body (black)? And at night time, white is probably the best for all hot temps? 67.165.185.178 (talk) 21:28, 13 December 2021 (UTC).

No. Black or white only changes the heat received from the Sun, but has little to no impact on other factors (body radiation and/or convection/conduction etc.). Therefore clothing color at night does not matter. For the day, you probably want white on hot days and black on cold days, but color matters much less than the fabric, flow of air around the piece of clothing etc.

TLDR: in winter, you should rather wear a white sweater than a black sundress. Tigraan^{Click here for my talk page ("private" contact)} 10:06, 14 December 2021 (UTC)

Without sunlight, the colour doesn't matter. With sunlight, black clothes will get warmer than white clothes – at least on the outside. But that doesn't necessarily mean they also get warmer on the inside, in particular when it's hot and you mostly rely on evaporation (sweat) for cooling. So if it's cold and sunny, wear black. If it's not sunny, wear whatever you like. If it's hot and sunny, it gets complicated. Even more so when you also take UV protection into account. If you've got very loose clothes flapping in the wind in a hot desert, black may be better than white, as it does a better job keeping you in the shade. You may have noticed that Arabian fashion can be black or white, but is always loose. If your clothes are not loose (sporting outfit, maybe), white is better than black, usually. PiusImpavidus (talk) 10:22, 14 December 2021 (UTC)

Organs of tinned fish

If I consume sardines or pilchards out of a can, I have so far always found that inside each fish, apart from the backbone and spinal cord, there is usually also either one of two things: eggs (i.e. roe) or what I assume is the liver, the latter of which I don't consume for reasons of accumulation of toxins. However I cannot recall finding both of these (i.e. liver and roe) together in any one fish. Is this a known 'thing'? Do female fish absorb their own livers while eggs develop within them? Am I misidentifying the liver (sometimes there appear to be two of these)? Why are other organs not apparent (e.g. heart, intestines etc.)? PaleCloudedWhite (talk) 16:49, 11 December 2021 (UTC)

Could be milt rather than liver. Delicious. Bazza (talk) 19:16, 11 December 2021 (UTC)

Sardine digestive apparatus - ventral view after dissection. Alansplodge (talk) 16:29, 12 December 2021 (UTC)

Thanks for the replies. It looks like some of the 'livers' I perceived are maybe testes instead, which would explain why they don't appear on fish that have visible roe. As to the other organs, it looks like they can be quite small, and maybe the cooking process diminishes them further? They must still be there, as it doesn't seem plausible that the canning process would remove some organs but not others. PaleCloudedWhite (talk) 20:00, 15 December 2021 (UTC)