Why is a hollow chamber with a small hole in it a good approximation a blackbody? question about this: is the small hole what makes the chamber hollow?

Explained

Explanation:

A small hole in a chamber can make it a black body. The small hole enables only small amount of radiation to pass through it and this small amount of radiation after multiple reflections inside the chamber is absorbed in the chamber itself.

Therefore making the chamber a black body. A black body is the one absorb all the radiation incident on it.

no sign of dia of fuel cell