A conical hole in a block of metal for which ε = 0.5 is 5 cm in diameter at the surface and 5 cm deep.
By what factor will the radiation from the area of the hole be changed by the presence of the hole?
(This problem can be done to a close approximation using the methods in this chapter if the cone does not become very deep and slender.
If it does, then the fact that the apex is receiving far less radiation makes it incorrect to use the network analogy.)