Learning goals

•     To understand the symmetry rules in action when light reflects off a mirror.
  
  

Summary

Any illuminated object reflects part of the light shining on it but only the polished surface of a mirror can render a perfect reflection. The light rays from an actual object bounce off the mirror to give a virtual image. With a flat mirror, the real object and the virtual object are symmetric and appear to be at the same distance on either side of the plane of the mirror.

Symmetry rules at work in the reflection process explain how an image is formed by a plane mirror.

Object A (a teapot) can be moved around, illustrating the symmetry rules at work in the reflection of light rays.

Click and drag the teapot.

Learn more

The light rays reflected off a plane mirror obey Snell-Descartes’ laws, which state that the angle of reflection of a light ray is equal to its angle of incidence.

A plane mirror is stigmatic, meaning that each point of the object corresponds to a single point of the image, which makes them perfectly symmetric with respect to the plane of the mirror. For the reflection to be perfect, the defects on the reflecting surface must be smaller than the wave length of the light (a few hundredths of a micrometer).
To amplify light, lasers use two flat mirrors set face to face, one being a half mirror which  allows rays of sufficient intensity to pass, and so be emitted outside the device.
Telescopes use large concave mirrors which focus all the light on the eyepiece or on a test instrument.