Optical telescopes may be divided into two general categories: (1)
refracting telescopes that use lenses to gather and focus light, and
(2) reflecting telescopes that use mirrors to accomplish the same
purpose. We discuss the principles of refracting telescopes in this section
and of reflecting telescopes in the following section.
Principle of Refraction
As we have discussed in earlier sections, the direction of light propagation is
changed at the boundary of glass and air by
By designing lenses having the right
curvature, this principle can be used to gather and focus light. The following
figure illustrates the use of a lens to gather and focus light, and the use of
two lenses to make a simple refracting telescope.
Principle of refraction and the refracting telescope
Here are Java applets illustrating
image formation by a
converging lens, and by a
diverging lens. These will give you some feeling for how such lenses are
used to image objects.
One problem with refracting telescopes is that there is a frequency dependence
for refraction, so the amount of refraction at each surface of the lens depends
on the wavelength. Thus, different wavelengths focus at slightly different
points. This is called chromatic aberration, and causes objects like
stars to be surrounded by fuzzy, rainbow colored halos. Chromatic aberration
can be corrected by using a second carefully designed lens mounted behind the
main objective lens of the telescope to compensate for the chromatic aberration
and cause all wavelengths to focus at the same point.