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So it is said that a perfect blackbody absorbs all the energy and then emits all of that in the form of blackbody radition. Why should it emit all the absorbed energy? Why can it not emit some and keep rest within?

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  • $\begingroup$ Since it absorbs all the wavelengths (in principle), every radiation arising from it must be emission instead of reflection. $\endgroup$ – exp ikx Jan 20 at 16:56
  • $\begingroup$ More on it here. $\endgroup$ – exp ikx Jan 20 at 16:59
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Why should it emit all the absorbed energy? Why can it not emit some and keep rest within?

It can, for a while. If it does so it will get hotter, emitting more, until it reaches equilibrium.

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Why should it emit all the absorbed energy.? Why can it not emit some and keep rest within.?

If an object were able to emit with a different emissivity than it absorbed (at a given frequency) then it could be used to transport energy at that wavelength against a temperature gradient. In other words it would violate the 2nd law of thermodynamics.

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Atoms in a body with temperature above zero Kelvin are subject of vibrations and during the accelerations of the involved subatomic particles they emit photons.

Image a closed volume with perfect (idealized) reflecting walls and inside two bodies of the same material, mass and form with different temperatures. The body with the higher temperature contains atoms with higher vibrations and by this emits photons, which in sum have of bigger energy as the stream of photons from the cooler body. Emitting and receiving photons, after a while both bodies have the same temperature. That is what happens around us all the time between all temperature differences and it is called thermal equilibrium.

Every material has its own characteristic of the emitted photons over the full spectrum of wavelengths.

enter image description here
Metal halid lamp spectrum

So it is said that a perfect blackbody absorbs all the energy and then emits all of that in the form of blackbody radiation.

A black body, as any body, temporarily absorbs the energy from all the photons which hits the body. If the body is cooler than the surrounding, it re-emits less energy as the body receives. If the body is hotter, it re-emits more energy as he receives.

Specific for a block body is its characteristic emission spectrum which depends only from its temperature (and not from its material):

enter image description here

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