The Obedient Reflected Ray
Two mirrors are joined at a fixed angle at . A ray of light shines into the angle between them, parallel to the horizontal mirror and at a given height. It bounces a number of times. Suppose it strikes the lower mirror at right angles at and then reemerges along the original path. This is called an obedient ray.[more]
What is the relationship between the distance between the height and the length of ? The answer is given in the Details.[less]
They are equal.
 D. Wells, The Penguin Book of Curious and Interesting Puzzles, New York: Penguin Books, 1992.