Let a lens image the hole in a cavity of area AH on a black object of area A0 Use an equilibrium argument to related the product AHH to A00 where H and 0are the solid angles subtended by the lens as viewed from the hole and from the object This general property of focusing systems is easily derived...

Assume that the solid angles H and 0 are centered about the directions perpendicular to the areas A ...

Let a lens image the hole in a cavity of area AH on a black object of

Question:

Let a lens image the hole in a cavity of area AH on a black object of area A0. Use an equilibrium argument to related the product AHΩH to A0Ω0 where ΩH and Ω0are the solid angles subtended by the lens as viewed from the hole and from the object. This general property of focusing systems is easily derived from geometrical optics. It is also true when diffraction is important. Make the approximation that all rays are nearly parallel.