Study of Black-body Radiation Why did scientists study black body radiations from something as complicated as a hollow container rather than the radiation from something simple like a thin solid cylinder?
 A: A black body is a perfect absorber, and in practice it's difficult to make a material that is a perfect absorber. So to make a black body we choose a material that is as good an absorber as we can find, and form it into a hollow sphere with a small hole in it. The black body is then the hole, not the sphere. It's a nearly perfect absorber because any radiation falling on it is multiply reflected within the sphere. Because the material of the sphere is a good absorber and absorbs most of the radiation at each reflection, the chance that any radiation falling on the opening reflects out again is vanishingly small.
I've never heard of a hollow cylinder being used to model a black body, but the hole at the end of the cylinder would be a black body in a similar way to the hole in the sphere.
A: Since a black body is an exact absorber and during practice it's demanding to make things that is really exactly a good absorber so to make black body we choose a material that is as good an absorber as we can obtain it into a hollow Sphere with a small hole in it. The black body is then the hole not the sphere. It is nearly perfect absorber because any radiation falling on it is multiply reflected within the sphere because the material of the sphere is a good absorber and absorbs most of the radiation at each reflection.
