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Plane Mirrors

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The Person In The Mirror

The Person In The Mirror

Credit: Jan (Johannes) Musset
Source: http://en.wikipedia.org/wiki/File:Droste.jpg
License: CC BY-NC 3.0

The image shows a woman holding an object that holds a smaller image of her, which holds an even smaller image of her holding the same object. This effect is known as the Droste effect, and it can be replicated in real life by standing in between two mirrors.

Amazing But True

Credit: See-ming Lee
Source: http://www.flickr.com/photos/seeminglee/2568041925/
License: CC BY-NC 3.0

The right hand is reflected as the left hand in the mirror [Figure2]

  • You will notice that when you wave your right hand in front of a mirror, the image you see looks like a left hand waving back. The reversal of your hands is because the front and the back of your hand are reversed by the mirror. This is known as depth inversion. In terms of the coordinate axis, the perpendicular axis that is pointing out of the mirror is simply reversed.
  • You can also see that when you stand in front of a mirror, the image looking back at you is the same distance inside the mirror as you are from the mirror. This is because after the light from you reflects off the mirror, the rays of light diverge exactly as if they came from a point behind the mirror that equals your distance in front of the mirror. Since your eyes cannot distinguish the difference between light actually diverging from a real object or the light from the mirror, the image in the mirror is known as a virtual image. 
  • Listen to the explanation of depth inversion by Dr. Richard Feynman at the link below: 

http://www.youtube.com/watch?v=6tuxLY94LXw

Can You Apply It?

Using the information provided above, answer the following questions.

  1. If you are standing 2 ft from a mirror, how far is your image inside the mirror?
  2. Assume you are standing between two mirrors. If you are 2 ft from each mirror, how far is the 1st image in front of you inside the mirror behind you?
  3. If you stood between two plane mirrors, how many images would you be able to count if you had an infinite amount of time to count all the images?

Image Attributions

  1. [1]^ Credit: Jan (Johannes) Musset; Source: http://en.wikipedia.org/wiki/File:Droste.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: See-ming Lee; Source: http://www.flickr.com/photos/seeminglee/2568041925/; License: CC BY-NC 3.0

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