Plane Mirror as the Screen in Young's experient If I place a plane mirror instead of the screen in the Young's double slit experiment, what will I see? Will the bands appear on the mirror face? 
 A: The reason we see an interference pattern on a screen is because of diffuse reflection. This is because in diffuse reflection, the incident light can be considered to be absorbed and uniformly emitted out in all directions. This results in a brightness at a point proportional to the brightness of the incident light. 
A mirror, however, simply reflects everything in a directed manner according the laws of reflection. What happens when you look at the mirror? You'll only see light that has directly been reflected to your eyes. In this case, you won't see the interference pattern itself, but only the part of it directly where you are observing it. You'll effectively see an image of the two slits (with the interference pattern determining the overall amount of light you see). But the pattern itself is formed everywhere in space; you can only see the full pattern when there is a diffusely reflecting surface or some other kind of 'absorbing' detector.
Keeping a mirror is equivalent (for the most part) to simply flipping the world behind it onto the world ahead. So if you have a screen in which an interference pattern is obtained, take a plane in front of the screen, and flip everything behind it over to the front, you will still get a screen with an interference pattern i.e. you will get an interference pattern if you keep a screen in front of the mirror. The only complication is that the world ahead (i.e. source and slits) remain where they are, and in the practical case, there will be some shadows, extra light from the source, and so on.
