Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

An alternative optical test

(f) An alternative test for finding the focal angle. The mirror is the same as in figure (d)

Question: Figure (f) shows an alternative optical test. Rather than placing the object at infinity as in figure (e), we adjust it so that the image is right on top of the object. Points O and I coincide, and the rays are reflected right back on top of themselves. If we measure the angle q shown in figure (f), how can we find the focal angle?

Solution: The object and image angles are the same; the angle labeled θ in the figure equals both of them. We therefore have θio = 2θ= θf. Comparing figures (e) and (f), it is indeed plausible that the angles are related by a factor of two.

Last Update: 2010-11-11