A camera could focus light with a single lens. A microscope objective could be built with a single lens. So why would anyone ever use a lens combination in place of a single lens? There are two primary reasons.

First, as you learned in the previous section, any lens has dispersion. That is, its index of refraction varies slightly with wavelength. Because the index of refraction for violet light is larger than for red light, a lens’s focal length is shorter for violet light than for red light. Consequently, different colors of light come to a focus at slightly different distances from the lens. If red light is sharply focused on a viewing screen, then blue and violet wavelengths are not well focused. This imaging error, illustrated in FIGURE 35.23a, is called chromatic aberration.

FIGURE 35.23 Chromatic aberration and spherical aberration prevent simple lenses from forming perfect images.

FIGURE 35.23 Full Alternative Text

Full Alternative Text

Second, our analysis of thin lenses was based on paraxial rays traveling nearly parallel to the optical axis. A more exact analysis, taking all the rays into account, finds that rays incident on the outer edges of a spherical surface are not focused at exactly the same point as rays incident near the center. This imaging error, shown in FIGURE 35.23b, is called spherical aberration. Spherical aberration, which causes the image to be slightly blurred, gets worse as the lens diameter increases.

Fortunately, the chromatic and spherical aberrations of a converging lens and a diverging lens are in opposite directions. When a converging lens and a diverging lens with different indices of refraction are used in combination, their aberrations tend to cancel. A combination lens, such as the one in FIGURE 35.23c, can produce a much sharper focus than a single lens with the equivalent focal length. This is the main reason that most optical instruments use combination lenses rather than single lenses.