Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|cite|improve this question
There is indeed, excellent intuition! This is called conservation of etendue. – user2963 Apr 8 '12 at 12:43
Thanks! If you post that as an answer, I will accept it. An addition: If I would use a solar cell and an electrical lightsource both highly effective, it should be possible to build the box. Is this possible becuase it's by non-optic means, or is there a deeper law that would limit the effectivenes of the cell or the source to impede this? – dronus Apr 8 '12 at 23:42
@dronus It is possible because transfer entropy somewhere else. In fact you need some energy to erase information, but its negligible. – Piotr Migdal Apr 9 '12 at 10:21
Perhaps @zephyr should submit an answer to this question so that this moves out of the 'unanswered' section? – Kitchi Nov 12 '12 at 13:08
BTW Because the entropy of light is entropy like any other, you can trade it for other kinds of entropy. So if you don't mind producing some heat as a side effect, you can concentrate light further than you can with lenses and mirrors if you interpret 'concentrate' liberally enough. See: – Dan Piponi Sep 24 '13 at 18:27
up vote 3 down vote accepted

(I'm copy-pasting from @user2963's comment into this community wiki.)

There is indeed, excellent intuition! This is called conservation of etendue.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.