- #1
the4thamigo_uk
- 47
- 0
Ive heard a black body described as 'a body which absorbs all the radiation that falls upon it'. This seems to me to contradict the idea that a black body emits exactly as well as it absorbs light.
I can understand that if you shine light matching a blackbody curve (at a given temperature) on a black body that it should be capable of absorbing all that light. Furthermore it would absorb the light as heat and immediately re-mit the exact same profile of light.
But imagine instead a profile of light that is greater in intensity than the blackbody curve for every wavelength. Indeed, consider a profile that does not even match the 'shape' of the blackbody curve i.e a flat distribution of light. The first definition would suggest that the black body would be able still to absorb all the light. Is this the case? If so, the temperature of the black body would rise and re-emit the light as a new blackbody curve for the higher temperature. If not, then are we saying a black body can 'reflect' some of the excess light that it is not capable of absorbing?
So, what do we mean when we say a black body absorbs as well as it emits?
Is it, that the black body can only absorb the exact same profile of light as it emits at a given temperature, or is it the weaker statement that the black body aborbs all profiles of light, but that it then re-emits a blackbody profile that has the same 'total power' (over all wavelengths) as the incident light?
I can understand that if you shine light matching a blackbody curve (at a given temperature) on a black body that it should be capable of absorbing all that light. Furthermore it would absorb the light as heat and immediately re-mit the exact same profile of light.
But imagine instead a profile of light that is greater in intensity than the blackbody curve for every wavelength. Indeed, consider a profile that does not even match the 'shape' of the blackbody curve i.e a flat distribution of light. The first definition would suggest that the black body would be able still to absorb all the light. Is this the case? If so, the temperature of the black body would rise and re-emit the light as a new blackbody curve for the higher temperature. If not, then are we saying a black body can 'reflect' some of the excess light that it is not capable of absorbing?
So, what do we mean when we say a black body absorbs as well as it emits?
Is it, that the black body can only absorb the exact same profile of light as it emits at a given temperature, or is it the weaker statement that the black body aborbs all profiles of light, but that it then re-emits a blackbody profile that has the same 'total power' (over all wavelengths) as the incident light?
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