Wikipedia:Reference desk/Archives/Science/2022 September 18

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September 18

Does IR emit heat?

Sunlight is 53% IR, and I don't see a regular light bulb emitting heat, so does the heat from the sun mainly come from IR? Also, looking at who absorbs and reflects IR and UV the most. When under a tree and in a forest-area, plants I believe are good absorbers of UV (as good as sunscreen?), but do they absorb or reflect IR? What about cement/concrete, do they absorb or reflect IR and UV? Thanks. 67.165.185.178 (talk) 15:27, 18 September 2022 (UTC).

Infrared radiation is the radiative manifestation of heat (as opposed to heat manifesting through conduction or convection). As such, it behaves as any other component of electromagnetic radiation, of which light is another component. Individual materials and media have different properties that concern the absorption, reflection or transmission of infrared radiation, just as they do for visible light. Most materials with high visible light reflectivity reflect infrared radiation to some extent. By the way, tungsten filament light bulbs are far more efficient in the radiation of infrared than visible light, hence the increased efficacy of LED light sources. Acroterion (talk) 15:34, 18 September 2022 (UTC)

So prior to the invention of LED light bulbs, were incandescent light bulbs slowly cutting down on the % of IR emission? 67.165.185.178 (talk) 16:00, 18 September 2022 (UTC).

To a certain extent. However, incandescent light bulbs operate using incandescence - the heating of a filament to such high temperatures that it emits bright light, requiring extreme temperatures. There are limits to the efficiency such a method can achieve. Acroterion (talk) 19:33, 18 September 2022 (UTC)

67.165.185.178, both the Sun and an incandescent filament are essentially black body radiators. As such the distribution of radiance by wavelength is dependent almost exclusively on the temperature. The only way to make an incandescent bulb "more efficient" in terms of visible-light-to-IR ratio is to make it hotter. And you can't really make them as hot as the Sun (almost 6000K), because that's well above the melting point of tungsten (actually within spitting distance of its boiling point).

In contrast, LEDs and even compact fluorescent bulbs generate light by an entirely different process, and can be tuned to emit the wavelengths you want by varying the composition.

Worth noting — while you may be right about the fraction of the Sun's radiance that's in infrared, the peak radiance is at about 500 nm, well into the green and shading towards blue. Here's a useful graph, taken from Wien's displacement law. Eyeballing it, it looks like it could be true that more than half the power is in the IR, but if so it's only because of the fat right tail of the distribution.

Side note to Acroterion -- calling infrared "the radiative manifestation of heat" is a bit misleading in general. To the extent that it's true, it's only because of the spectra of black bodies in the range of temperatures familiar to humans. Otherwise there's nothing specially "heat-like" about IR compared with other wavelength bands. Not saying you didn't know that, just clarifying for the possible benefit of other readers. --Trovatore (talk) 20:39, 18 September 2022 (UTC)

I agree, I oversimplified in order to point out that heat or the perception of heat is transferred or manifested in different ways. Humans generally perceive IR as "heat" since it tends to heat people or objects according to their absorption of the energy. Other electromagnetic radiation can do the same thing, such as microwaves in ovens (through a somewhat different process) or x-rays in hydrogen bombs. Acroterion (talk) 21:05, 18 September 2022 (UTC)

It's about half infrared, per hertz and per nanometer on a log scale and half the watts above this nanometer and half the photons above this nanometer and there's others have different peaks. One of them is (maybe photon count) about 850 nanometers. Another (harmonic or something, I forgot what it's called) is per inch of spectrum from a prism instead of per nm of wavelength. Sagittarian Milky Way (talk) 00:02, 19 September 2022 (UTC)

(edit conflict) Infrared radiation, like all electromagnetic radiation, carries energy. When such radiation is absorbed, this energy normally heats up the surface on which it falls. (In photosynthesis it is converted to chemical energy.) IR is special in that it is readily absorbed by many surfaces; conversely, hot surfaces up to about 4000 K emit heat in the form of electromagnetic radiation with a peak in the infrared part of the spectrum; see Thermal radiation. Virtually all surfaces reflect at least some of the incident light of any given wavelength and let some pass through when transparent and sufficiently thin, reflecting the remainder. Concrete does heat up in bright sunlight, mainly because of the IR it absorbs.  --Lambiam 15:58, 18 September 2022 (UTC)

• Incident infrared radiation is very good at causing molecules to vibrate; a property we know as "heat". Other ranges of the electromagnetic spectrum are not so efficient at that; for example ultraviolet radiation doesn't really cause molecular vibration, but what it can do is break bonds between the atoms of molecules, which is why UV radiation is known to cause skin damage and cancer. We actually take advantage of the ability of molecules to absorb IR in the form of Infrared spectroscopy. --Jayron32 17:34, 19 September 2022 (UTC)

I'm seeing somewhere that incandescent light bulbs are mostly red light with some IR, and very little blue. But since they look white/yellow light to me, then that means incandescent light has a range of wavelengths, rather than a thin range of wavelengths? (That neutralize to yellow-looking light?). Then what about incandescent Christmas light bulbs that are red and blue, are those narrow-range of wavelengths? Or it's just some filter? 67.165.185.178 (talk) 02:16, 23 September 2022 (UTC).

I wondered about this as a 5 year old. If you removed the glass will you find a colored bright string or normal? It's normal. Sagittarian Milky Way (talk) 09:28, 23 September 2022 (UTC)

So if you measured the wavelengths from a incandescent red to blue light bulb (or wavelength distributions) it would be the same? 67.165.185.178 (talk) 11:54, 23 September 2022 (UTC).

Incandescent bulbs are a very good approximation of blackbody radiation, which is to say that the range of temperatures you see is essentially perfectly dependent on temperature. --Jayron32 12:17, 23 September 2022 (UTC)

Red or blue incandescent light bulbs get their color from the color of the glass used for them. The black body radiation assumes clear glass. NadVolum (talk) 17:18, 23 September 2022 (UTC)

I mean white and yellow light bulbs emit white and yellow light, but for colorless glass? It doesn't seem red to me. 67.165.185.178 (talk) 22:34, 23 September 2022 (UTC).

Our brains adjust for changing white balance throughout the day, and by extension into artificial light. It used to be easy to illustrate this with a film camera. Two exposures, one in daylight and one under incandescent light would have radically different colour profiles. Martin of Sheffield (talk) 11:47, 24 September 2022 (UTC)

Can you get the raw electron counts from digital cameras and do the white balance yourself in Photoshop or some other software? And see what the raw mosaic of red, green and blue squares looks like at exactly 4 monitor pixels per camera subpixel? Sagittarian Milky Way (talk) 12:41, 24 September 2022 (UTC)

Essentially, yes. Good cameras offer both jpg and raw formats, and the latter can be colour balanced in software such as Photoshop and Lightroom. They can also have the exposure adjusted by a stop or two.--Phil Holmes (talk) 13:05, 24 September 2022 (UTC)

What is a Lachnite gem?

Lachnite is mentioned in many, many ads in US pulp magazines as an artificial diamond, hard to tell from the real thing. One ad lists the hardness as 9.4, the melting point as 5050 F, color as blue-white, and refractive index as 2.105, from the Harold Lachman Company, at 1/30th the cost of diamond and 30 times the cost of cheap imitation diamonds. (Details from https://archive.org/details/bm_1922_08/page/n1/mode/2up, in a 1922 issue of Black Mask magazine.) Does anyone know what this is, or might have been? -- Prosfilaes (talk) 22:45, 18 September 2022 (UTC)

The advert says it was invented by "M. Verneuil, the world-famous scientist". His process was the Verneuil process. That article mentions both rutile and Strontium titanate as diamond substitutes made by the process. DuncanHill (talk) 22:59, 18 September 2022 (UTC)

Their articles say rutile was first made in 1948 and strontium titanate as a diamond substitute in 1955. Maybe it was spinel? In any case, good catch on the name.--Prosfilaes (talk) 02:22, 19 September 2022 (UTC)

There are also mentions of "colourless corundum", also called "white sapphire",^[1] which can also be produced by the Verneuil method.^[2] Such white sapphires have reportedly occasionally been passed of as diamonds.^[3] However, the physical properties of spinel and corundum do not match those cited for lachnite. Spinel is much softer (Mohs scale 7.5–8.0). Corundum melts at 3,711 °F. Both have refractive indices less than 1.8.  --Lambiam 09:40, 19 September 2022 (UTC)

Based on its physical properties, cubic boron nitride (c-BN) would be a candidate for being the identity of the mysterious "lachnite". However, known by the brand name Borazon, it is said to have been first produced in 1957. I also did not find any suggestion that c-BN can be synthesized with the Verneuil process.  --Lambiam 10:18, 19 September 2022 (UTC)

It definitely does sound like that. It isn't Borazon as produced by whatever proces that uses as Borazon isn't colorless, pure c-BN should be colorless and that's what the ads were claiming. Perhaps they were using some process that has been lost? NadVolum (talk) 21:35, 19 September 2022 (UTC)

I had a look up on the web and it seems Harold Lachman had a brother Dr Arthur Lachmam who was a chemist. He invented the Lachman process for purifying petroleum in 1929. I don't see anthing about him doing work on boron nitride but he seeems to have done some work with the nitrogen compounds of zinc. The Lachnite gems were sold between 1916 1n 1922 and in 1920 he went missing for six months with loss of memory but was fine afterwards. To get anything definitive I think you'd have to ask their relatives as there is very little on the web. NadVolum (talk) 16:17, 23 September 2022 (UTC)

Moissanite is another possibility but it doesn't match quite so well - and it also suffers from the problem of a good process not being developed anytime near then - but at least they are produced as diamond substitutes nowadays. NadVolum (talk) 16:52, 23 September 2022 (UTC)