So, for example: if I wore a black undershirt beneath a long sleeved shirt or jacket, would I be warmer than if I wore a white shirt? I know that you can “see” heat with infrared goggles, but are infrared light and heat basically the same thing?
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Heat can be radiated as black body radiation, that is infra red radiation and therefore photons aka light, juts a lower wavelength.
But regula heat transfer is done by molecules bumping into each other.
So the suns light is black body radiation, just super intense. And it heats your shirt, but a black shirt will reflect less light so it will absorb more heat.
But heat and infra red is not allways the same.
This is a bit complicated, mostly because a pigment can be black as far as visible light is concerned, but completely transparent to infra-red light.
Secondly, colour only affects radiated heat but when it comes to clothing conduction and convection are also important. There is unlikely to be much radiative transfer between your skin and clothing – this will be dominated by conduction. In effect it is only the colour of the very outer layer that matters.
Also you need to consider which way the heat is going; generally from your body to the atmosphere. As such you want your clothes to minimise heat transfer if you want to stay warm. In that respect white clothing would be preferable to black (with the caveats above).
Heat is infrared radiation emitted by energetic matter. Infrared is also light, at a frequency beyond the red end of the spectrum. Our eyes can’t detect that light frequency. As a result, humans and warm blooded animals glow in that spectrum. We’re literally luminous, but aren’t aware of it.
Yes, black absorbs heat because it also absorbs almost all frequencies of light, allowing none of the colour frequencies to reflect back off it. So technically a black shirt underneath would be warmer, although thin fabric materials don’t absorb heat and store it very efficiently IIRC.
It’s a misconception that “infrared light = heat”. *Any* object with heat will radiate some kind of light depending on its temperature. It just happens to be the case that at human temperatures, the light is infrared.
The sun is very hot, and as a result gives off a lot of visible light. If everything were surface-of-the-sun temperatured, then we’d think of visible light as being equal to heat.
The reason black shirts warm you up is because they absorb the visible light from the sun. Absorbing energy of any kind will heat you up, so the black shirt heats up.
If I had to guess, there would be no noticeable difference between wearing a black shirt vs a white shirt under your coat.
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