102 PART 1^ |^ EXPLORING THE SKY
Focal lengthFocal length
Light reflects from a
metal film and does
not enter the glass.Short-focal-length
lenses and mirrors
must be strongly
curved.Rays of light traced through the lensObjectObjectImageImageLight focused by a
lens is bent to form an
inverted image.Light focused by a concave mirror
reflects to form an inverted image.Light rays from a distant source
such as a star are nearly parallel.■ Figure 6-4
You can trace rays of light from the top and
bottom of a candle as they are refracted by
a lens or refl ected from a mirror to form an
image. The focal length is the distance from
the lens or mirror to the point where paral-
lel rays of light come to a focus.
(or objective) mirror—a concave piece of glass with a refl ective
surface—forms an image by refl ecting the light. In either case,
the focal length is the distance from the lens or mirror to the
image of a distant light source such as a star. Short-focal-length
lenses and mirrors must be strongly curved, and long-focal-
length lenses and mirrors are less strongly curved. Grinding the
proper shape on a lens or mirror is a delicate, time-consuming,
and expensive process.
Th e image formed by the primary lens or primary mirror of
a telescope is small, inverted, and diffi cult to view directly.
Astronomers use a small lens called the eyepiece to magnify the
image and make it convenient to view (■ Figure 6-5).
Refracting telescopes suff er from a serious optical distortion
that limits their usefulness. When light is refracted through glass,
shorter wavelengths bend more than longer wavelengths, so blue
light, for example, having shorter wavelengths, comes to a focus
closer to the lens than does red light (■ Figure 6-6a). Th at means
if you focus the eyepiece on the blue image, the other colors are
out of focus, and you see a colored blur around the image. If you
focus on the red image, all the other colors blur. Th is color sepa-
ration is called chromatic aberration. Telescope designers can
grind a telescope lens of two components made of diff erent kinds
of glass and so bring two diff erent wavelengths to the same focus
(Figure 6-6b). Th is does improve the image, but these achro-
matic lenses are not totally free of chromatic aberration. Even
though two colors have been brought together, the other wave-
lengths still blur. Telescopes made with achromatic lenses were
popular until the end of the 19th century.
Th e primary lens of a refracting telescope is more expensive
than a mirror of the same size. Th e lens must be achromatic, so
it must be made of two diff erent kinds of glass with four precisely
ground surfaces. Also, the glass must be pure and fl awless because
the light passes through it. Th e largest refracting telescope in the
world was completed in 1897 at Yerkes Observatory in Wisconsin.