Measuring speed of light I have an interesting thought:
could we use recursion between two mirrors to measure speed of light? It seems to be not really hard experiment. How could we do it? I'm not sure about how the picture inside mirrors propagates.
 A: You are right, you can use the bouncing of a light beam between mirrors to measure the speed of light. This was done many years ago by the physicists Fizeau and Foucault to get the first accurate measure of c, a simplified description of the test apparatus is as follows: 
We position a pair of mirrors a known distance apart and shine a beam of light so it bounces between them. Then we place a spinning wheel with slots in its periphery between the mirrors so it chops up the beam as the wheel spins.
Then we spin the wheel up from zero RPM to a very high speed. At some point, the wheel is spinning so fast that a beam of light that makes it through one slot to bounce off the mirror and come back towards the wheel will be blocked because the slit has rotated out of alignment with the beam in the amount of time it took for the beam to make a round trip to the mirror and back. 
Knowing the length of that path and the speed of the wheel, you can then calculate the speed of the light beam. 
Note that the real apparatus was more complicated than this, but the experiment was a success.
