Is there something like an entropy law for light rays?
I came up with the following experiment: A black box has two circular holes in it, a small and a large one. I don't care about there placement. Now I like to build something into the box that any ray entering the large hole will exit the box through the small hole.
However, I could not imagine any combination of optical tools like lenses, mirrors and so on that would do that.
It seems possible to concentrate a parallel bundle of light to a small spot or hole, but not unordered rays.
Is there a law written in ink that forbids such a contraption?