When a light ray bounces off a mirror it is reflected so that the angle of incidence is the same as the angle of coincidence. In the figure below we see a light ray traveling from a target to an observer by way of a mirror. Notice that the angle that the portion of the light ray from the target to the mirror makes with the mirror is the same as the angle the portion of the light ray from the observer to the mirror makes with the mirror. The observer, being used to light rays that travel in straight lines assumes that the incoming light ray is an unbent straight line and, thus, it appears to the observer that the target is located at the apparent position shown in the figure below. This figure is a Java applet produced by Geometer's Sketchpad. You can drag the point representing the observer or the point representing the target to see what happens to the apparent position of the target.
This page uses JavaSketchpad, a World-Wide-Web component of The Geometer's Sketchpad. Copyright © 1990-2001 by KCP Technologies, Inc. Licensed only for non-commercial use.