Experiment 12 ~ Optics
Introduction
Light rays are bent upon moving between media of different indices of refraction. Additionally,
light rays can be reflected from surfaces. These two properties of light propagation cause images
to appear distorted from their original form. This shape change can come in the form of
enlargement or reduction in size (magnification), inversion, and even anisotropic “stretching”.
The changes in images due to the bending of light rays allow you to view distant and
microscopic objects, which otherwise could not be seen. (That’s why lenses were so important in
the development of modern science!)
The refraction of light and the production of images are demonstrated in this experiment. First,
the effects of refraction will be studied and quantified by experimentally measuring the focal
lengths of various lenses. Additionally, the thin-lens equation,
,
will be used as a second method of obtaining the focal length of a lens, this time by calculation
rather than by direct measurement. As you saw in lecture, the thin-lens equation allows you to
find image locations (di) for lenses that have a thickness much less than the focal length f. Using
the thin-lens approximation, a geometric study of light rays may be used, as opposed to the
wave-based approach required in the previous experiment (Diffraction and Interference). In the
last part of the experiment, the principles studied in the first part of the lab will be applied to
construct a basic telescope.
Part 1 – Measuring Focal Length
Equipment
Here, you will need a meter stick with a stand, a screen, a 5W light bulb with meter stick holder,
and three lenses (10-cm, 15-cm and 30-cm).
Procedure
In this part of the experiment, the focal lengths of convex, converging lenses will be measured.
This method is possible with these types of lenses because they produce real images which may
be projected onto a screen, as opposed to a virtual image which cannot be projected onto a
screen.
Start with the first lens:
 Place the lens in the lens holder and attach it to a meter stick. Place the light source on one
side of the lens, about a meter and a half away. (You may need to share meter sticks with
other lab groups!) This approximates a situation where the object distance is equal to infinity.
 Set the voltage of the power supply for the light source to zero. Connect the alligator clips to
the light, and then turn on the power supply. Turn up the voltage slowly until you reach about
2-3 Volts. Don’t go any higher, or you’ll blow the light bulb!
 Place the screen on the meter stick, on the opposite side of the lens from the light. Move the
screen until you see a clear, sharp image of the light. Adjust the screen until you make the
image as sharp as possible. The distance between the lens and the screen is the focal distance.
Record this value in Data Table 1. Note that since the light source (the object) is far from the
lens, we can approximately say that do=∞ in the thin lens equation. Then, according to the
thin-lens equation, the image is focused at a distance of one focal length from the lens, i.e.,
di=f.
 Repeat this procedure twice. The average of the three trials will be used as the focal length of
the lens. Follow this procedure for each of the lenses, as listed in Data Table 1. The focal
lengths you have measured should be close to the values stated for each lens (10, 15 or 30
cm).
Data Table 1
Focal Length, f
(lens 1)
Focal Length, f
(lens 2)
Focal Length, f
(lens 3)
Trial 1
Trial 2
Trial 3
Average
Now, we are going to measure the object distance do (which won’t be equal to infinity this time!)
and the image distance di for the 10-cm lens. From those measurements, we will use the thin-lens
equation to calculate f, and compare it to the value measured above.
• Place the lens in the middle of the meter stick and the light source at a distance of roughly six
times the focal length away from the lens (in other words, do=6f). For the focal length, use
the value you measured above from Table 1 – ignore the value given by the manufacturer,
since this is not exact.
• Place the screen on the opposite side of the lens from the light source. Move the screen until
you find the position (di) where the image is projected in focus onto the screen. Record the
•
image and object distances in Data Table 2, and calculate f using the thin lens equation.
Repeat this procedure for each of the object distances given in the first column of Data Table
2.
To demonstrate the thin-lens relationship, make a plot of the inverse object distance (1/do) on
the x-axis versus the inverse image distance (1/di) on the y-axis. The y-intercept of this line
should be the focal length, according to the thin-lens equation. (Think mathematically about
why this has to be true!) How does this value compare to the focal lengths calculated from
the thin lens equation in each row of Data Table 2?
Data Table 2
Object Distance Image Distance do
do
Focal
Length
Equation,
f
from
Lens
6f =
4f =
3f =
2f =
1.5f =
Intercept value from plot ___________________
Part 2 – Build a Telescope!
Equipment
Here, you will need a meter stick, 2 lens holders, a 10-cm lens, a 15-cm lens and a 30-cm lens.
Procedure
In this experiment, you will make a basic telescope!
A refracting telescope in its most basic form consists of an objective and eyepiece lens. The
objective lens focuses parallel light rays from a distant source to produce an image, whereas the
eyepiece lens magnifies this image, allowing it to be viewed by the eye. This basic procedure
will be followed here to make two telescopes. The 30-cm lens will serve as the objective lens
both cases, while the 10-cm and 15-cm lenses will be used independently as eyepieces to provide
two different magnification examples.
 Place the 10-cm lens about 10 centimeters from the end of the meter stick. This will serve as
the eyepiece lens.
 Next, place the 30-cm lens on the meter stick at a distance of 40 centimeters from the
eyepiece lens. The distance of 40 centimeters is used, because the image formed by the
objective lens will be used as the source of the eyepiece lens. Therefore, the two focal lengths
are added, giving 40 centimeters.
 Looking through both lenses, using the 10-cm lens as an eyepiece lens, view a distant object
through both lenses. Be careful to keep the lenses on the meter stick!
 Gently adjust the eyepiece lens position until the image of the object is clear and focused.
Everyone's eyes are different and the eyepiece must be adjusted individually. Also, the focal
lengths of the lenses may not exactly match the values provided by manufacturer. Therefore,
a separation of 40 centimeters is simply a starting point.
 What do you observe about the image you see?
 Repeat the experiment with the 15-cm lens. What is different from the case with the 10-cm
lens?
Questions
1. Could you have done this experiment with diverging lenses rather than converging?
2. How does your measurement of the focal length for each lens (Table 1) correspond to the
value given by the manufacturer? How can you explain the difference?
3. How does your measurement of the focal length of the 10-cm lens in Data Table 2
compare to the average value you found from Table 1? Which way of measuring f do you
think is more accurate? Why? How do those values compare to the one you determined
from the intercept of the plot?
4. What do you notice about the image you see through your telescope? Can you estimate
how much the image is magnified? Is it magnified differently when you replace the 10cm lens with the 15-cm lens?
SOMETHING TO THINK ABOUT: What could you do differently to build a microscope
instead of a telescope?