(question by Henry Greenside, Duke Physics) Consider three identical square plan
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Question
(question by Henry Greenside, Duke Physics)Consider three identical square planar mirrors that are glued together to form three adjacent sides of a cube meeting at a corner, with the mirrored sides all facing towards each other. Show that these mirrors act as a universal reflector that sends light back to its source: any light beam entering this arrangement of mirrors will leave parallel and opposite to its original direction.
If you look into such a corner reflector, what kind of image will you see of your face?
Note: Such corner reflectors were left on the surface of the moon by Apollo astronauts and were used in ranging experiments, in which laser beams from an observatory on Earth were bounced off the surface of the Moon and returned to the observatory, with the time of transit being measured. This enabled the distance from the Earth to the Moon to be measured with high accuracy which has been useful in testing the theory of general relativity and also for investigating the geology and origin of the Moon.
Explanation / Answer
You might want to protect the mirrors by putting them in the corner of a wooden or cardboard box. Plastic mirrors were recommended by one of the web pages. If you do use glass, you might want to tape all the exposed edges to reduce the risk of cuts. If you don't use a separate base, putting both of the vertical mirrors on top of the horizontal mirror would give it more support. Front (or first) surface mirrors are best but are probably harder (and more expensive) to get. The reflective material is not behind the distorting glass or plastic. Because the reflective surface is exposed, it is more likely to get damaged.
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