Laboratory Exercise 3
In the laboratory exercise students will observe image formation and
determine the focal length of a thin lens using two different methods.
They will also observe aberrations.
Equipment:
He-Ne laser
Diffuse reflecting screen
Converging lens
Mirror
Lamp
Optical track
Ruler
Experiment 1:
Determine the focal length of a converging lens!
Procedure:
Place the lens in its holder on the track.
Turn on the laser and shine it onto the diffuse reflecting screen.
You should see a red spot.
Make sure the red spot and the center of the lens are at the
same height, and as you look along the track, the spot is aligned
with the middle for the lens.
This red spot will be your object. If this object is located in the
focal plane of the lens, then light from the object will leave the
lens as a parallel beam. If a parallel beam is incident on the
lens, it will come to a focus in the focal plane. This is called the
principle of ray reversibility.
Place the mirror approximately 25 cm away from the screen.
Make sure the center of the mirror is at the same height as the
red spot and the center of the lens. The mirror will reflect a
parallel beam of the light and focus it back to a small spot.
Move the lens until you obtain the smallest reflected spot
possible. Adjust the mirror so that the reflected spot appears
just above the object spot.
Measure the distance from the center of the lens to the screen
(in units of cm). That distance is your measured focal length of
the lens. Record your measured focal length in the table below.
Check if moving the mirror a small distance along the track
changes anything.
Rotate the lens approximately 20 degrees about the vertical axis in
either direction. Describe what happens to the reflected spot. You
should observe a lens aberration called “coma”. It is observed when
the light rays make large angles with the optic axis.
Experiment 2:
Observe the formation of a real image using a converging lens!
Procedure:
Turn off the laser and place the lamp on the track as shown.
Make sure the lamp and the lens are at the same height.
Place the lens approximately 25 cm away from the lamp on the
track.
A black cross is painted onto the lamp. Move the screen along
the track until you see the sharpest image of the cross on the
screen. Measure the object distance xo from the cross on the
lamp to the center of the lens and the image distance xi from the
center of the lamp to the image on the index card and record
these distances in the table below.
Measure the height of the cross ho and the height of its image hi
and record these heights in the table below.
Note if the image is upright or inverted.
Using the lens equation,
1/xo + 1/xi = 1/f, calculate the focal length and using the
equation M = -xi/xo calculate the magnification.
Repeat these measurements with the lens placed approximately
30 cm and 35 cm away from the lamp.
Again rotate the lens approximately 20 degrees about the vertical axis
in either direction. Describe what happens to the image. Are the
vertical and horizontal lines still in focus at the same place? If not,
move the screen to put either the vertical or the horizontal line of the
cross in focus. You should observe a lens aberration called
“astigmatism”.
Experiment 3:
Your lens is a plano-convex lens. One side is flat and one side is
curved. With the lens approximately 25 cm away from the lamp, look
into the lens from the lamp side. With the curved side facing you, you
should see a virtual image of the cross produced by reflection from the
front surfaces of the lens. Move the lens back and force and try to
"touch" the images with your finger. Is it in front or behind the lens?
With the plane side facing you, you should see a real image of the
cross produced by reflection from the back surfaces of the lens. Move
the lens back and force and try to "touch" the images with your
finger. Is it in front or behind the lens? Observe!
Experiment 1: measured focal length:
Experiment 2:
measured measured calculated measured measured measured
calculated
xo
xi
focal
ho
hi
magnification: magnification:
length:
M = -hi/ho
M = -xi/xo
f=
xoxi/(xo +
xi)
Laboratory 3 Report:
Name:
E-mail address:
In a few words, describe the experiment.
Comment on the procedure. Did you encounter difficulties or
surprises?
Present the table with your results.
Comment on your results. For example:
Do the measured and the calculated focal length agree
within experimental uncertainties?
Do the measured and the calculated magnification agree
within experimental uncertainties?
Did you observe an inverted or an upright image?
Did you observe aberrations?
What did you observe when you looked for the images
produced by reflection?