If all we were interested in was how rays of light behaved, the previous chapter would be the end of the discussion. However, we use light to see things, and that adds a level of complication.

How the Eye Sees Things

We know that rays of light are emitted radially from sources of light, including non-luminous object that reflect light diffusely . Our eye is quite aware of this fact, and uses it to locate objects in space. When diverging rays of light enter the eye, the eye will trace those rays back to a single point, and conclude that those rays were emitted from an object at that location.

This works great, most of the time. But the eye can be fooled. Specifically, if the rays of light are diverted between the object and the eye, made to change direction, than the eye will deduce that they came from a completely different location, and see an object where there is no object at all. What the eye sees is an image of the object that created those rays. This is an optical illusion of the basest sort, so fundamental that we don't even think of it as a trick.

Object Underwater

For instance, suppose a light is placed in a cup of water. When its rays reach the surface of the water and enter the air, they are refracted away from the normal. When an eyeball sees those rays, it will trace them back to an image that is closer to the surface than the object is. This is why swimming pools look shallower than they really are, and you can try it yourself with a penny in a cup of water.

Plane Mirror

Images you see in a mirror are created in a similar way. When an object is placed in front of a plane mirror (that is, one that isn't curved), rays of light from the object are reflected backwards according to the law of reflection. When the eye traces those rays backward, they appear to become from a point behind the mirror: a rather bizarre conclusion, but one we take for granted.

This figure shows the rays of light from an object after they pass through some unknown device. Where will an image of the object appear?
Optional: Use geometry to prove that the image in a plane mirror is just as far behind the mirror as the object is in front of it.