Two plane mirrors are facing each other. They are parallel, d = 8 cm apart, and l = 18 cm in length, as the drawing indicates.

Two plane mirrors are facing each other. They are parallel, d = 8 cm apart, and l = 18 cm in length, as the drawing indicates. A laser beam is directed at the top mirror from the left edge of the bottom mirror. What is the smallest angle of incidence with respect to the top mirror, such that the laser beam