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?
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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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