Laser reflected between two mirrors. © Exploratorium, www.exploratorium.edu
The simple law of reflection says that the angle of incidence equals the angle of reflection. Thus, as exhibited in this photo, if a laser is shone between two mirrors which are not quite parallel, but slightly bent toward one another, the laser’s reflection will eventually double back and return in the direction from which it came.

Laser reflected between two mirrors.
© Exploratorium, www.exploratorium.edu

The simple law of reflection says that the angle of incidence equals the angle of reflection. Thus, as exhibited in this photo, if a laser is shone between two mirrors which are not quite parallel, but slightly bent toward one another, the laser’s reflection will eventually double back and return in the direction from which it came.