Light, Sound and Waves

Seeing yourself in the mirror

Physics Narrative for 11-14 Supporting Physics Teaching

Mirror images

What is going on when you look at your own face in the mirror? Put simply, light beams from every point on your face travel to the mirror, where they are reflected. Some of those reflected beams will travel towards your eyes where they will be detected. The image that you see has two interesting features:

  • The image of your face appears to be behind the mirror. Indeed the farther you step back away from the mirror, the farther behind the mirror the image is.
  • The image of your face that you see in the mirror seems to be the wrong way round. For example, if you hold your right ear and look in the mirror, you see your own image with your hand holding onto your left ear. Check it!

How can we explain these two features? With a ray diagram!

From the diagram you can see that the image (from which the rays appear to come) is as far behind the mirror as the object (face) is in front. You can also see that right and left seem to be reversed on the image. The apparent reversal of right and left is referred to as lateral inversion.

Reading in reverse

You may have noticed that ambulances and police cars sometimes have writing or electronic signs with the letters reversed on the front of them. The reason for this is so that drivers in front of these vehicles can, when they look in their rear view mirror, see the writing the correct way round.

is formalised by Law of Reflection
can be exhibited by Progressive Wave
has the special case Total Internal Reflection
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