# Hi guys! One question is troubling me a lot. It is know fact that a hollow metallic enclosure with a small hole is a perfect black body. When electromagnetic waves of any frequency are incident on the hole, it is completely absorbed after a number of reflections. So, the condition for absorptivity=1 is satisfied as required for it to be a perfect black body. But it is said that the inner walls of the cavity should be maintained at constant temperature. What is the purpose of having this?