Figure 49.8: Optical principle of the compound microscope.(Credit: “Hyperphysics,” Georgia State Uni-
versity, http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html))
Reflecting telescopes are made in a number of different designs (Fig. 49.9). The simplest is theNewtonian
reflector. In this design, light enters the telescope tube, reflects from a large parabolicprimary mirror,upto
a flatsecondary mirror, and from there out through the side of the tube to an eyepiece.
In aCassegrain, light reflects from a large parabolic primary mirror to ahyperbolicsecondary mirror;
from there it travels back through a hole in the center of the primary mirror to the eyepiece.
An interesting property of reflecting telescopes is that they produce aninverted(upside-down) image.
This is not a problem for astronomical observations, but this means that using a reflecting telescope to make
terrestrial observations requires the use of animage erector— an optical device placed at the eyepiece to
make the image rightside-up.
A number of reflecting telescopes have been placed in space, either in Earth orbit or elsewhere. There are
a number of reasons for placing a telescope in space:
- The Earth’s atmosphere is a fluid with turbulent air currents that tend to blur images in ground-based
telescopes. By placing the telescope above the Earth’s atmosphere, the telescope no longer need “look”
through the atmosphere, so the images are much sharper and more detailed. - The Earth’s atmosphere absorbs many wavelengths of light. A telescope in space can observe at wave-
lengths that are impossible for a ground-based telescope. - Since the sky is always dark in space, a space-based telescope can make observations at any time —
unlike a ground-based telescope, which can only make observations at night.
49.7 The Periscope
Aperiscope(from the Greek , “around”, and
o!, “see”) is an instrument designed to allow the
user to observe above or around a barrier. Most famously, periscopes are used in submarines to observe above
the water while the submarine remains submerged. In its simplest form, a periscope consists of two mirrors
mounted at 45 ıangles: one at the top, and one at the bottom, where the observer is located (Fig. 49.10).
If you have seen a submarine periscope in use (in real life or in a movie), you will notice that in order
to look around, the periscope operator rotates the entire instrument by walking around in a circle. Why not