This optics problem doesn't have an answer in my book, so I can't really check whether the answer is right; screencap of problem posted below momentarily.

Question

mendicant_bias · Answer

http://i.imgur.com/bd5xsOQ.png

anonymous · Answer

I think the way this is approached is that plane mirrors simply reflect rays at an angle equal to the angle to that of the incident ray.

So initially, it comes in parallel to the symmetry axis. Using alternate interior angles you can see pretty easily that the angle between the incident ray and the mirror is 30-degrees. This means that the reflected ray will be 30-degrees to the surface of the mirror as well.

After this ray reaches the other mirror, you'll notice you have a triangle which has a 30 and 60-degree angles. In order for the triangle to have 180-degrees, the unknown angle of incidence must be 90-degrees. This means the ray will reflect directly back along the path it came.

When it reaches the point on the first mirror it initially reflected off of, it simply continues to trace its path along which it came.

Therefore, it reflects 3 times and leaves at the same place it entered (obviously in the opposite direction).

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