telescopes used a combination of two lenses to make distant objects appear both nearer and
larger. The termtelescopewas coined by the Italian scientist and mathematician Galileo
Galilei (1564–1642). Galileo built his first telescope in 1608 and subsequently made many
improvements to telescope design.
Telescopes that rely on the refraction, or bending, of light by lenses are calledrefracting
telescopes,or simplyrefractors.The earliest telescopes, including Galileo’s, were all refrac-
tors. Many of the small telescopes used by amateur astronomers today are refractors with
a design similar to Galileo’s. Refractors are particularly good for viewing details within our
solar system, such as the surface of Earth’s moon or the rings around Saturn.Figure23.4
shows the biggest refracting telescope in the world.
Figure 23.4: The largest refracting telescope in the world is at the University of Chicago’s
Yerkes Observatory in Wisconsin. This telescope was built in 1897. Its largest lens has a
diameter of 102 cm. ( 20 )
Around1670,anotherfamousscientistandmathematician—SirIsaacNewton(1643–1727)—built
a different kind of telescope. Figure23.5shows a telescope similar in design to Newton’s.