PHYS 2212 Module 14.1

Images Formed by Plane Mirrors

Recommended Reading

14.1 Images Formed by Plane Mirrors

Learning Objectives

By the end of this section, you will be able to:

  • Describe how an image is formed by a plane mirror.
  • Distinguish between real and virtual images.
  • Find the location and characterize the orientation of an image created by a plane mirror.


Image in a Plane Mirror

If light rays come from a “point source” (a small bulb, the tip of my nose, the end of an arrow, … a particular point on an object) and then reflect off a planar, flat mirror, they will appear to come from behind the mirror. 

My eye sees (some of) the light rays leaving the object. My brain assumes the rays must have traveled in straight lines, so my brain “draws the dashed lines” and deduces the object must be located at the “image point” shown on the right.

This image is called virtual. The rays appear (to me) to come from that point in space, but they were never really there. If you put a piece of paper somewhere back there blocking the dashed lines, it has no effect on the image. The paper is behind the mirror, after all!

We’ll encounter real images soon, like the image projected onto a movie screen. But you don’t get that with a simple flat mirror – as you can see above, the image is virtual.

In this movie I show how you are able to see an image in a flat mirror:


Practice 14.1.1
How tall must a “full-length” mirror be, relative to a penguin, so that he can see himself from hat to foot?
Check your answer: C. The mirror must be 1/2 as tall as the penguin.