You will remember the spectrum that is produced when light goes through a prism. The prism bends or refracts the light-but bends or refracts light of different colors different amounts. This is fine if you want the colors of a spectrum. In that case, the more dispersion there is the better things are. But if you are trying to produce a sharp image, any dispersion at all will spoil it.

Figure 18.21 shows parallel white light being focused by a lens. Just as with a prism, violet light will be bent more than red light-and all the other colors of the spectrum will lie between these two extremes. This means the violet light will be focused closer to the lens and red light will be focused farther away. If we place a card at the location of the violet focus, we will see colored shadows or halos around it with red on the outside. If we place a card farther away, at the location of the red focus, we will see colored shadows or halos around the central red dot with violet on the outside. This problem with image formation is called chromatic aberration. We can correct for chromatic aberration by replacing a simple, single-element lens with a lens made of two or more pieces of glass that have different dispersion characteristics.

We can correct for chromatic aberration by replacing a simple, single-element lens with a lens made of two or more pieces of glass that have different dispersion characteristics. Modern camera lenses are always multi-element designs to correct for this and other aberrations. Mirrors do not have chromatic aberration; light of different colors still behave exactly the same under reflection. For several reasons, most astronomical telescopes use mirrors instead of lenses. One very important reason for this is that reflecting telescopes do not need to be corrected for color.

As larger and larger spherical lenses or larger and larger spherical mirrors are used, we find that light that comes in farther and farther from the optic axis is bent more and focused closer to the lens or mirror. This is shown in Figures 18.22 and 18.23. This is known as spherical aberration and is simply the result of geometry. For a mirror, this aberration may be corrected if the mirror is ground to be part of a parabola of revolution instead of a sphere. Such mirrors are called parabolic mirrors and all reflecting astronomical telescopes will have parabolic mirrors. The problem is the same for spherical lenses-lenses whose surfaces are parts of spheres. The problem may be corrected in lens design by going to a multi-element lens with different elements made of glass with differing characteristics. Another solution with lenses, is to go to parabolic lens elements which are usually known as aspheric lenses. These problems become greater as the lens gets larger and this explains why large aperture lenses which are great for low-light conditions are so very much more expensive than small aperture lenses which take great pictures outdoors in the Sun.

Q: Almost all large, modern astronomical telescopes are reflectors that use mirrors rather than refractors that use lenses. Why might that be?

A: Mirrors do not have chromatic aberration. A mirrors has only a single surface to grind instead of two for a lens. Mirrors can also be supported from the back by something like a steel frame.