Why does this mirror always reflect light back at its source?

Question

When Apollo astronauts landed on the moon, they left behind a mirror so that laser could be used to accurately measure changes in the moon earth distance. This mirror was made two mirrors with a 90 degree angle between them that was pointed at earth. Light incident on this mirror would undergo successive reflection before being reflected back.

Why this mirror would always reflect light back at its source?

Answer 1

There is a physics theorem stating that the angle of incidence, of a reflected wave (such as a light wave), is equal to the angle of departure. Assume that the laser is emitted at right angle from a big plate that if you bring that plate close to the mirrors they will form a right triangle in which the plate will the the hypotenuse.

The laser hits the first mirror at an angle x, then it will leave at angle x and will hit the other mirror at an angle (180-90-x=90-x, a triangle is formed between the corner (90o) that joins the two mirrors and the first angle of departure and the second angle of incidence) then the second angle of departure will be 90-x and will hit back the plate at a right angle.  The diagram below hopefully helps to visualize this:

http://img527.imageshack.us/img527/1720/...

The light comes back to same direction albeit parallel to the original ray. You may notice that all triangle angles add to 180 and the rectangle angles add to 360.

Answer 2

Light travels in perfectly straight lines. It goes where its pointing really lol