The source in this figure is above the plane of the mirror's surface. It is from a vertical plane on which the light will be detected, something usually called a "screen" but it can be in freely open space. Two paths are shown, the direct one to the screen, and a reflected path taking a longer distance and more time to reach the screen. The reflected path appears to come from another "virtual" slit that is below and behind the mirror as seen from the screen. You could imaging removing the mirror and having another source at that virtual point and have the same geometry for the light. This is simply the geometry for the two-slit or double slit interferometer. 1. Explain why the separation of the two slits is 2