Note above that the bicycle reflector retro-reflects the laser beam back directly to the laser (it may be better visible in the reflection on the table top). Try this reflector and compare it to using a plane mirror to do the same. How is this reflector constructed? Give a mathematical proof/explanation for the retroreflector.
Retro-reflectors were placed on the moon to reflect light back to the earth in this manner. The distance to the moon could be measured to within a few centimeters and monitored as a function of time to check predictions of the General Theory of Relativity!