Question

On the + y axis a laser is located at y = + 3.0 cm. The coordinates of a small target are x = + 12.0 cm and y = + 6.0 cm. The + x axis represents the edge-on view of a plane mirror. At what point on the + x axis should the laser be aimed in order for the laser light to hit the target after reflection?

Answer 1

The angle to the x-axis must be the same for the incident and reflected ray:

$$y_1/x_1=y_2/(x_2-x_1),\\ 3/x_1=6/(12-x_1),\\ x_1=4\text{ cm}.$$