On the front of many ambulances, why is the word “ambulance” printed backwards?
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The answer is a matter of angles. The law of reflection states that the angle of light incident on a mirror must be equal to the angle of the reflected light. If you are standing directly in front of a mirror, the angle of incidence (that is, the angle between the direction the incoming light is traveling and the mirror surface) might be 90° and as a result reflects directly back to your eyes at an angle of 90°. If, however, you stand off to one side of the mirror, then the angle of reflectance is much smaller, and so the incident light won’t come from your face, but from another part of the room.
The word “ambulance” is printed backwards (left and right reversed) so that when viewed in a mirror—specifically, the rearview mirror of a car—it will appear correctly (that is, forwards), ensuring that the driver can read it and respond as quickly as possible.