I found this on Wikipedia article on black bodies:
A black body in thermal equilibrium (that is, at a constant
temperature) emits electromagnetic radiation called black-body
radiation. The radiation is emitted according to Planck's law, meaning
that it has a spectrum that is determined by the temperature alone
(see figure at right), not by the body's shape or composition.
Now I'm a bit confused. Isn't the black body radiation given by $\frac{P}{A}=\sigma T^4$
So it doesn't depend on body color, composition or anything else than its size.
Shouldn't the radiation be the same for all bodies, but we use the black body because its a convenient model because all of its radiation comes from emission and not reflection or anything else?
Best Answer
A real object will radiate less energy than a black body at the same temperature. The ratio of the emission to the black body emission is called the emissivity. The Engineering Toolbox web site has data on emissivities for a range of materials. For example a polished silver surface has an emissivity of 0.02 i.e. it radiates only 2% of the power that a black body of the same temperature radiates.
The emissivity is related to the reflectivity by E + R = 1. A black body reflects none of the light falling on it so R = 0 and E = 1. Anything that has a non-zero reflectivity must have an emissivity of less than 1 otherwise it could be used to build a perpetual motion machine.