the lens are imaged farther from the lens than rays passing through points near the edges.
Figure 36.10 earlier in the chapter showed a similar situation for a spherical mirror.
Many cameras have an adjustable aperture to control light intensity and reduce
spherical aberration. (An aperture is an opening that controls the amount of light
passing through the lens.) Sharper images are produced as the aperture size is
reduced because with a small aperture only the central portion of the lens is exposed
to the light; as a result, a greater percentage of the rays are paraxial. At the same time,
however, less light passes through the lens. To compensate for this lower light intensity,
a longer exposure time is used.
In the case of mirrors, spherical aberration can be minimized through the use of a
parabolic reflecting surface rather than a spherical surface. Parabolic surfaces are not
used often, however, because those with high-quality optics are very expensive to make.
Parallel light rays incident on a parabolic surface focus at a common point, regardless
of their distance from the principal axis. Parabolic reflecting surfaces are used in many
astronomical telescopes to enhance image quality.
Chromatic Aberrations
The fact that different wavelengths of light refracted by a lens focus at different points
gives rise to chromatic aberrations. In Chapter 35 we described how the index of refrac-
tion of a material varies with wavelength. For instance, when white light passes through a
lens, violet rays are refracted more than red rays (Fig. 36.35). From this we see that the
focal length of a lens is greater for red light than for violet light. Other wavelengths (not
shown in Fig. 36.35) have focal points intermediate between those of red and violet.
Chromatic aberration for a diverging lens also results in a shorter focal length for
violet light than for red light, but on the front side of the lens. Chromatic aberration
can be greatly reduced by combining a converging lens made of one type of glass and a
diverging lens made of another type of glass.
S E C T I O N 3 6 . 6 • The Camera
1153
Violet
Red
Red
Violet
F
V
F
R
Figure 36.35 Chromatic aberra-
tion caused by a converging lens.
Rays of different wavelengths focus
at different points.
Quick Quiz 36.9
A curved mirrored surface can have (a) spherical aberra-
tion but not chromatic aberration (b) chromatic aberration but not spherical aberra-
tion (c) both spherical aberration and chromatic aberration.
36.6 The Camera
The photographic
camera is a simple optical instrument whose essential features are
shown in Figure 36.36. It consists of a light-tight chamber, a converging lens that
produces a real image, and a film behind the lens to receive the image. One focuses
the camera by varying the distance between lens and film. This is accomplished with an
adjustable bellows in antique cameras and with some other mechanical arrangement in
contemporary models. For proper focusing—which is necessary for the formation of
sharp images—the lens-to-film distance depends on the object distance as well as on
the focal length of the lens.
The shutter, positioned behind the lens, is a mechanical device that is opened for
selected time intervals, called exposure times. One can photograph moving objects by
using short exposure times or photograph dark scenes (with low light levels) by using
long exposure times. If this adjustment were not available, it would be impossible to
take stop-action photographs. For example, a rapidly moving vehicle could move
enough in the time interval during which the shutter is open to produce a blurred
image. Another major cause of blurred images is the movement of the camera while
the shutter is open. To prevent such movement, either short exposure times or a
tripod should be used, even for stationary objects. Typical shutter speeds (that is,
exposure times) are (1/30)s, (1/60)s, (1/125)s, and (1/250)s. For handheld cameras,
Shutter
Lens
Aperture
Film
Image
q
p
Figure 36.36 Cross-sectional view
of a simple camera. Note that in
reality, p ++ q.