Conceptual Physics

The parabolic design is used not just for visible light but for other forms of

electromagnetic radiation, like radio waves, as well.

For instance, the dish antennas used for satellite television are small parabolic

reflectors. These antennas do not have to be as smooth as light reflectors because the

wavelength of the television signals they receive is far greater than the wavelengths of

visible light. For electromagnetic radiation of even longer wavelengths, like the longest

radio waves, the receiving parabolic reflectors can be even less smooth, but they must

be very wide. You see an example of a large radio telescope in Concept 3.

Mirrors having a precise parabolic shape are difficult to manufacture. Some modern

optical telescopes, called mosaic telescopes, combine the best features of both

spherical and parabolic mirrors. They consist of a number of spherical mirror segments

arranged in an overall parabolic shape. The mirrors are cut into hexagons so that they

fit together neatly with no gaps. In the photograph above you see such a mirror.

This arrangement works well because the individual spherical segments are small

enough to have negligible aberrations of their own and to be easy to assemble, and like

all spherical mirrors they are easy to manufacture. Yet each small mirror can be

positioned as part of a large overall parabolic shape that optimizes the net image quality

produced by the reflector.

Spherical mirrors

Suffer from spherical aberration

Parabolic mirrors

Do not exhibit spherical aberration

Parabolic reflectors in use

Common in telescopes and satellite

dishes

31.11 - Ray-tracing fundamentals

Ray tracing enables you to determine the fundamental properties of an image: Is it upright or inverted? Smaller or larger than the object? Real

or virtual?

Ray tracing also provides an effective tool for verifying the computational results you get from applying the mirror and lens equations. It is easy

to drop a sign, forget a reciprocal, or make other errors when applying these equations; ray tracing allows you to subject your answers to a

“reality check.”

In this section, ray tracing will be used to determine the basic attributes of an image that is produced by an object located farther from a

concave mirror than the center of curvature. You see this configuration above.

What will be the nature of the

image?

(^582) Copyright 2007 Kinetic Books Co. Chapter 31