Lab 6: Lenses, Mirrors and Telescopes
In this lab you will
investigate some of the properties of lenses and mirrors and see how they can be
used to construct telescopes. Recall that there are two basic types of
telescopes – refractors and reflectors. A refracting telescope uses a lens to
focus the collected light. The lens that collects and focuses the light is
called the objective lens or simply the objective. A reflecting
telescope uses a curved mirror, called the primary mirror or simply the
primary, to focus the light. Once the light is focused, a second lens must
be used as an eyepiece to view an image. There are several different styles of
refracting and reflecting telescopes. In this lab you will construct two styles
of refracting telescopes, an “astronomical telescope” that is like the
telescopes you use in lab, and a “Galilean telescope” that is like the telescope
Galileo used and is also like the “spyglass” used by sailors. You will also see
how a curved mirror, like in a reflecting telescope, can focus light.
Light passing
through a piece of glass is bent or refracted. Lenses are shaped to take
advantage of this effect; they focus light to a particular point, called the
focal point. The distance from the lens to the focal point is called the
focal length. (See the figure.)
In
lenses like the one above, called converging lenses, incoming light is
focused to one point. Converging lenses are thicker in the middle than the
edges. In a diverging lens incoming light diverges (spreads out) from
the focal point. Diverging lenses are thinner in the middle than the edges.
The objective lens of a refracting telescope is made using a converging lens,
but both converging and diverging lenses can be used for eyepieces. The
astronomical telescope uses a converging lens for the eyepiece, and the Galilean
telescope uses a diverging lens for the eyepiece.
PART
I: ASTRONOMICAL TELESCOPE
Find the focal
length of the large converging lens, which will serve as your objective. Do
this by mounting it in its holder on the optical bench and pointing the bench
out a door or window. Mount a card behind the lens and move it until a sharp
image of a distant object is seen. The distance between the lens and the image
(on the card) is the focal length. Record this distance. Repeat this procedure
for the small converging lens, which you will use as an eyepiece. Now move the
eyepiece to one end of the optical bench. Place the objective in a second lens
holder and mount it on the bench. Position the objective so that the distance
between the objective and eyepiece equals the sum of their two focal
lengths. Focus your telescope on a distant object out the door or window. You
may need to adjust the positions of the lenses slightly to obtain a sharp focus.
1. What were the
focal lengths you measured for your objective and eyepiece lenses?
2. What was the separation of the
eyepiece and objective when your image was in focus?
3. Is your image erect (right side up) or
inverted (upside down)?
4. Estimate the magnification of your
telescope by estimating how many times larger the image looks than the real
object.
5. The magnification of a telescope can
be calculated by this formula:
magnification =
objective’s focal length ÷ eyepiece’s focal length
Use this formula to calculate the
magnification of your telescope.
6. Compare the two magnifications in
questions 4 and 5. Are they reasonably close?
PART
II: GALILEAN TELESCOPE
Replace the
eyepiece lens with a diverging lens. The focal length of the diverging lens is
10 cm. Position the objective so that the distance between the objective and
eyepiece equals the difference of their focal lengths. This creates a
Galilean telescope. Focus on a distant object.
7. What was the
separation of the eyepiece and objective when your image was in focus?
8. Is your image erect or inverted?
9. Estimate the magnification of your
telescope by estimating how many times larger the image looks than the real
object.
10. Calculate the magnification of your
telescope (like you did in question 5 before).
11. Compare the two magnifications in
questions 9 and 10. Are they reasonably close?
PART
III: FOCUSING MIRRORS
Place the mirror
in a lens holder and point it out the door. Find the focal length by placing a
card in front of and off to the side of the mirror and moving it until you see a
sharp image of a distant object. The distance between the mirror and the card
is the focal length. (If we could place an eyepiece in front of the mirror as
we did with the refracting telescopes, we would have a reflecting telescope.)
12. What was the
focal length of your mirror?
13. Was the image formed by the mirror
erect or inverted?