Simplifying heavily, Heat is essentially the kinetic energy of the molecules and atoms of a material. It's transferred radiatively at all wavelengths, but short

Simplifying heavily, Heat is essentially the kinetic energy of the molecules and atoms of a material. It's transferred radiatively at all wavelengths, but shorter wavelengths carry more energy per photon. When those photons are absorbed by a material, the kinetic energy (and thus its temperature) rises. If you want to minimize radiative transfer, you have to maximize the amount of light reflected in the spectrum that's transferring energy and minimize absorption. When something is white, it's diffusely reflecting the entire visible spectrum. That works pretty well for a house in a sunny place, but you don't want to paint the mirrors in your optical cavity white though because there's only a narrow spectrum of light that needs extremely high reflectivity to be usable. For those we use specialized mirrors that can achieve much higher reflectivity over narrow ranges tuned to the application.