Pop Quiz: Michelson Interferometer

Inspired by one of yesterday’s easy questions, a pop quiz for you. The figure below shows a Michelson Interferometer:


A laser falls on a beamsplitter, which allows half of the light to pass straight through, and reflects the other half downward. Each of those beams then hits a mirror that reflects it directly back where it come from. The beams are recombined at the beamsplitter, and then fall on the viewing screen at the top of the figure.

When we add together the light from the two paths, we find that if the lengths of the two arms (that is, the distance from beamsplitter to mirror) are exactly the same, then we see a bright spot on the viewing screen that is as bright as the original laser (this is called “constructive interference”). if we move the right-hand mirror back by one half of the wavelength of the laser, we see no light at all on the viewing screen (“destructive interference”).

Here’s the question: When there’s no light on the viewing screen, we have light going into the interferometer, but no light coming out. What happened to the light?

Feel free to elaborate on your answer in the comments, or suggest additional answers that I may have left off.

(Interferometer picture grabbed from Stony Brook.)