Corner Reflector
This applet shows how light is reflected from a corner mirror. If the angle of the mirror is 90 degrees, the reflected ray will be parallel to the incident ray no matter what the incident angle is. You can the incident angle by moving the red dot at the right end of the incident ray. You can change the angle of the mirror by moving the dot at its corner. You can lengthen the mirror is necessary by moving the dots at its edges.