Question

Describe a simple thought experiment to prove that blackbody radiation is isotropic; that is,
the observed spectral brightness of a blackbody disc is independent of its orientation.

Answer 1

If the blackbody was not isotropic, we could use lenses, mirrors etc., to collect the radiation in the ‘bright’ direction and redirect it onto the ‘dim’ direction (or location) on the blackbody. Since this would put more energy into the object, it wouldn't be in thermal equilibrium anymore.