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Lab 8 Lenses!
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Objective!
The purpose of this lab is to examine lenses and optical instruments.!
Background!
Lenses follow the thin lens equation. Here, p is the object distance from the lens, q is the image
distance from the lens, and f is the focal length of the lens.!
1 1
1
+ = !
p q
f
The magnification can be shown to be!
q
M =− !
p
Combination of lenses next to each other have their focal lengths add in this way.!
1
1
1
+ = !
f1 f2
f
When multiple lenses are used in an optical instrument, You treat each lens one at a time. The
image from the previous lens is the object for the next lens.!
Experiment 1: Converging Lens!
We will measure the focal length of a converging lens. For a converging lens the focal length is,
by convention, positive.!
Place a light source on one end of the optical bench. Locate the actual object to be projected.
Place a 10 cm converging lens onto the optical bench so that it is 15 cm from the object.!
Look through the lens. What do you see? Is the view of the object larger or smaller? Is the view
of the object inverted? Place the screen onto the optical bench and find the location where the
image of the object is sharp. !
Record the following. The object distance is the distance from the lens to the object. The image
distance is the distance from the lens to the image. Calculate the focal length using the thin lens
equation.!
Increase the object distance by steps of 5 cm and repeat the process 4 more times. Find the
average focal length. Is it close to the advertised focal length?!
Experiment 2: Diverging Lens!
We will measure the focal length of a diverging lens. For a diverging lens the focal length is, by
convention, negative.!
A real object will not produce a real image that can be projected onto a surface. Look through the
diverging lens. What do you see? Why can you see the image clearly even though it can not be
projected onto a surface?!
To measure its focal length, we will have to use a converging lens to re-converge the image. We
will use the combination lens equation as well as the thin lens equation for this.!
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Tape together, at the edge, the 20 cm diverging lens and the 10 cm converging lens. Place the
light source 25 cm from center of the combination. Look through the lens. What do you see now?
Is the view of the object larger or smaller? Is the view of the object inverted? Place the screen
onto the optical bench and find the location where the image of the object is sharp.!
Record the following. The object distance is the distance from the combination lens to the object.
The image distance is the distance from the combination lens to the image. Calculate the focal
length of the combination lens using the thin lens equation. Calculate focal length of the diverging
lens using the combination lens equation.!
Increase the object distance by steps of 5 cm and repeat the process 4 more times. Find the
average focal length of the diverging lens. Is it close to the advertised focal length?!
Experiment: Keplerian Telescope!
A keplerian telescope is made from two converging lenses. The lens that the eye looks through
directly is called the eyepiece. The lens in front closest to the object is called the objective.!
Place a 5 cm converging at the edge of the optical bench closest to you. This is the eyepiece.
Place a 25 cm converging lens about 20 cm from the eyepiece. Look through the eyepiece and
adjust the position of the objective until you can see a clear image of something on the other side
of the room. !
Estimate the magnification of this telescope. What is the orientation of the image.!
Record the positions of both lenses and the object you are looking at. Use the thin lens equation
to find the image from the objective lens. Use the position of this image as the object for the
eyepiece. Calculate the magnification from both lenses and multiply them together. What is the
total magnification from this telescope?!
Experiment: Galilean Telescope!
A galilean telescope is made from one converging lens and one diverging lens. The eyepiece is
still converging lens. Use a 25 cm diverging lens as the objective. Again, adjust the distance
between two lenses until you get a clear image. !
Estimate the magnification of this telescope. What is the orientation of the image.!
Record the positions of both lenses and the object you are looking at. Use the thin lens equation
to find the image from the objective lens. Use the position of this image as the object for the
eyepiece. Calculate the magnification from both lenses and multiply them together. What is the
total magnification from this telescope?!
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