**Real images**

On the previous page, we assumed that the light rays
approaching the mirror were all parallel. That is true for light from a distant object,
but not for light from an object that is only a finite distance away from the mirror.
We now derive the point at which
the light is collected, the **image distance**, as a function of the distance
the object is separated from the mirror, the **object distance**. Consider an
object of height * h_{o}* and an image of height

The similarity of the green and aqua triangles allows one to write

One can combine these two equations, eliminating the ratio of the object and image heights, to obtain a relationship between the distances and focal length.

This is a remarkably useful formula. We have derived it for a concave mirror; but it will also apply to convex mirrors as well as to lenses.