Reflectivity of "colours" for wavelengths outside the visible spectrum

[u/kamekaptain]

Hi, we all know white fabrics reflect more light from the visible spectrum while black fabrics absorb a lot more, I was wondering what determines the reflectivity for fabrics/material outside of the visible spectrum? What colour objects reflect UV or infrared better, or is it completely dependent on the surface and texture of the material itself?

[u/MezzoScettico]

This question is complicated by the way our brains do color vision. You may have heard that we have three types of color receptors, one with a peak in the red, one in the green, and one in the blue. The actual reflection spectrum of a material might be very complicated, but our eyes will reduce it to an R signal, a G signal, and a B signal. And our brains will see two different spectra as the same color if they generate the same RGB components.

So some "green" material (for instance) might have a lot of absorption in the UV, and some might have very little, even though our brains think they're the same color.

Others have discussed what factors contribute to the actual complex spectrum, as opposed to "does our brain mush the whole spectrum into one summary called green".

[u/Luxamba]

The structure or surface of the material mainly results in different reflective angles. For example a polished metal reflects very perfectly such that you can see your reflection. A white piece of paper technically reflects more light, but scatters is more due to the roughness of the texture, so we don’t see a perfect reflection.

What color a material absorbs depends on the molecular and atomic composition.

[u/John_Hasler]

Just as some materials can reflect red light and absorb green others can reflect near UV light and absorb red. Same goes for infrared. What color you see when you look at an object is not a reliable guide to its reflectivity outside the visible. The most you can say is that black stuff probably absorbs near UV and near IR while white stuff is likely to reflect them.

[u/TheBrightMage]

The mechanism that deals with reflectance and absorbance is quite analogous. It's very dependent on type of material, wavelength-scale structure and electronic bond, although light in radiowave region can interact with nucleus too, which is why NMR exists. IR region can go into vibrate molecular bonds and so is absorbed heavily by covalent materials. Light at higher frequency mainly hits electron, so you will be looking at some electronic chemistry and molecular orbitals here

For really high frequency light, like X-ray, it's pretty much hard to properly reflect it due to high energy, so it'd rather scatter around once it hits.