Light wave question?

Question

Ok, say you combine 2 identical laser beams together; one way would be to have the two of them angled at 90 degrees to each other, forming an X, and place a half-silvered mirror angled at 45 deg. to both at the intersection. You'd still have an X, but one side would be half as bright. OK, as long as the 2 beams are in phase the 2 dimmer beams would each be as bright as 1 laser alone, but what if you moved 1 laser so it was 180 deg out of phase with the other? What would you see happen?

Answer 1

You will not be able to notice an interference of two laser beams of independent sources. This is a problem of quantum coherence.

If you want to do that, you will have to split a laser beam first. This way you can get two coherent beams. If you combine the beams with a half-silvered mirror, you will be able to notice interference. What you get is the Mach-Zender interferometer.

If you change the phase for one of the beams, you can change the interference.