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Does white give off heat when It reflects light? Can white absorb heat at all or just reflect all wavelengths?overall does white just reflect or can it absorb too?

Anan12345
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The total radiation emitted by an object is related to its temperature $T$ by the Stefan-Boltzmann law:

$$ J = \varepsilon\sigma T^4 $$

In this equation $\sigma$ is a universal constant called the Stefan-Boltzmann constant while $\varepsilon$ is a parameter called the emissivity that varies from object to object.

The emissivity is where the colour of the object comes in because white or polished silver objects have a low emissivity and black objects have a high emissivity. The emissivity has a maximum value of one and a minimum value of zero.

The emissivity is related to the reflectivity $R$ by:

$$ \varepsilon + R = 1 $$

So a white object with a high reflectivity and low emissivity reflects lots of radiation that falls on it and doesn't emit much. A black object with a low reflectivity and high emissivity reflects very little of the radiation that falls on it and emits relatively more radiation.

The emissivity (and therefore the reflectivity) are functions of wavelength so an object that has a low emissivity at visible wavelengths does not necessarily have a low emissivity at infra-red wavelengths. That means it's possible for a white object to be a good emitter of infra-red even though it is a poor emitter at visible wavelengths. The dependance of the emissivity (and reflectivity) on wavelength will depend on the structure of the material (mainly the electronic properties) so there is no universal law about how white objects behave.

John Rennie
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  • That is an excellent answer! Could I please get you to explain using emissivity, reflectivity and wavelength , the difference in how white and polished silver reflect. Thanks – Bill Alsept Jan 07 '16 at 08:15
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    @BillAlsept:see What is the difference between a white object and a mirror? and What is the color of a mirror?. If you need more than the answers to these questions provide it would be best to post a new question. – John Rennie Jan 07 '16 at 08:29
  • Thanks, I did read that one. I found the answers to be all over the place. One common point was that white light reflected in every direction but doesn't Feynman in QED say a mirror does the same thing? Anyway what I was really interested in was how you would explain it with emissivity, reflectivity and wavelength. Thanks – Bill Alsept Jan 07 '16 at 08:41
  • @BillAlsept: in white materials light is multiply scattered while in a mirror it is reflected once. That is the principle difference rather than anything related to the emissivity. A powdered mirror would look white and a powder fused into a glassy sheet would look like a mirror. – John Rennie Jan 07 '16 at 08:48
  • OH thanks a lot John now I'm off on another tangent, This is interesting and I need to think about about for a while. Does this metallic fusing at the edge of the glass allow photons to loop around those atoms on the edge that would normally be exposed to another medium?? Thanks – Bill Alsept Jan 07 '16 at 09:01
  • @Bill: [chat] is for discussion. Q&A is for Q&A, Anan's Q has been A'd. Comments are to help improve Qs or As, not for follow-up discussion. Please use chat or ask a separate (well formed) Q. – RedGrittyBrick Jan 07 '16 at 09:34