Geometrical Optics: Reflection and Mirrors

Normally we have no problem to decide where to put a beach umbrella to keep the sun out of our eyes, and we do not ponder about the electric field, the wavelength and the frequency of light.
This means that for many simple problems it is sufficient to concentate on the

light rays

the lines that describe in a simple geometric way the path of light propagation.

Geometrical Optics

is the study of those phenomena that can be understood by a consideration of the light rays only. Geometrical optics is useful as long as the objects with which the light interacts are much larger than the wavelength of the light.
In geometrical optics light does not `bend around corners'. One thinks of light as traveling in straight lines as long as it is left alone.

If an obstacle blocks some of the light rays headed for a screen, the light rays that are not blocked still reach the screen and make that part of the screen bright. Those that are blocked don't reach the screen, so the places where they would have hit are dark, a

Shadow

One can determine the location of the shadow by drawing straight lines from the point source of light to the edge of the obstacle and continuing them on the screen. We are familiar with reflected light, just as we are familiar with a ball reflecting (bouncing) off a wall. This common phenomenon,

Reflection

can be treated by geometrical optics. Light is travelling, say in air, in a straight line when it hits a different medium, say a mirror. Its direction changes suddenly, the old ray stops and a new one starts.

A central issue is to put together optical systems that deliver clear, undistorted images. Mirrors (and lenses) used in such system are most economically and simply manufactured if their surfaces have spherical shapes. Therefore, we need to discuss spherical mirrors.

Mirrors

Ch. Elster
Aug 26 14:27:03 EDT 2019