Physic 214 Laboratory
Interference and Diffraction
Interference of Light
Theory: Light that is incident upon two or more slits will interfere and produce an
interference pattern on a distant screen. If coherent light with wavelength  is incident
upon two slits the light will interfere constructively and produce maxima on a distant
screen when
d sin  = m 
(see Figure 1) where d is the slit separation, m is
an integer (m= 0, 1, 2, ...), and the angle  is given
by
tan  = y/D
where y is the distance along the interference
pattern, and D is the distance between the slits
and the screen (see Figure 1). (Note, on your
output, the intensity will be on the y-axis, and the
distance y will be along the x-axis.)
In this lab we will study the interference of light
that passes through two slits.
When light passes through a single slit, it is
diffracted. The diffraction of light superimposes
an envelope upon the pattern produced by the
interference of light, as shown in Figure 2.
Figure 2. The expected constant peak for the intensity
every time constructive interference occurs is modified
by the single slit diffraction pattern to produce the
pattern shown above.
Figure 1. a) A schematic of the
arrangement for obtaining interference
from a double slit showing the
distances and other parameters needed
for the equation of interference.
b) A schematic showing the path
difference between ray 1 and ray 2.
Note that D >> d so the rays are nearly
parallel.
Procedure and Calculations: Set up the laser light source, Multiple Slit Set wheel,
screen, and light sensor as shown in Figure 3. In this experiment, we will study the
interference patterns produced by 4 different double slits. These are labeled "Double
Slits" on the "Multiple Slit Set" wheel.
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Figure 3 The experimental apparatus with the multiple slit wheel in place.
The inputs to the Science workshop interface box are hidden by the label.
1). First, you need to set up Data Studio. Turn on the Science Workshop box, and then
turn on the computer. Go to Start, Physics, Data Studio, and Create Experiment.
Select Rotary Motion Sensor, and plug it into ports 1 and 2. Select Light Sensor, and
plug it into port A. Click on the General, and change the frequency to 50 Hz. Click on
the Rotary Motion Sensor tab and select 1440 in the Divisions/Rotation box. Click on
Measurements, and select Position. Save these settings.
Click on the Light Sensor. Click on Measurements. Select Intensity. Save these
settings by clicking OK. Click on the graph icon and drag in to Light Intensity. Click
and drag the position icon to the X-axis of the graph, so you have a graph of Intensity
versus position.
2). You need to make sure all of the equipment on the optical bench is set properly.
Before turning on the laser and before placing the Multiple Slit Set wheel on the optical
bench, set the slit width for the slit just in front of the light sensor to setting number 2.
Set the gain on the Light Sensor to 100. Center the aperture apparatus on the linear
translator and adjust the laser so it is incident in the middle of the slit in front of the
Light Sensor. Do this using the two knobs on the back of the laser (only make
adjustments using the knobs labeled "vertical" and "horizontal"). Place the Multiple Slit
Set wheel on the optical bench close to the laser and adjust the slit wheel so the laser
light is incident on the slit. If necessary adjust the laser again so the light strikes the
center of the aperture slit. Select the first of the 4 double slits that you are going to
study. Make sure that the interference pattern is bright and clear. If it is not, adjust
the wheel so the pattern is bright and clear. The laser should only be on when it is
necessary. Otherwise, turn it off. Do not look at the laser light directly.
3). Before you begin to record data, rotate the wheel around to see the different types of
interference and diffraction patterns produced by the different slit sets on the wheel.
4). You are ready to record data. Start with the intensity pattern off to the side on the
white screen. Set the wheel on the first of the four double slit sets you are going to
study. Press the Start button. Very slowly, move the wheel behind the Light Sensor to
move the interference pattern across the Light Sensor. When you are done, press the
Stop button.
5). Click on "Scale to Fit," which is the icon on the graph in the upper left hand corner.
You should see the intensity pattern inside the envelope of the diffraction pattern.
6). Use the "Smart Tool" (which is the icon that looks like a coordinate graph) to
accurately record the coordinates of several of the maxima and minima of the pattern.
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[To be able to completely control the Smart Tool, double click on the graph, then click
on the Tools tab and find the Gravity setting on the upper left. Set the Gravity to “0”.
Click OK until you exit the window(s)] Print out the graph.
7). Record the wavelength of the light, and the distance between the slit and the screen.
8). Use your observations to determine the separation between the two slits. Compare
this with that printed on the Multiple Slit Set wheel. Discuss the intensity pattern, and
the diffraction pattern. How does the slit width, a, affect the envelope around the
intensity pattern?
9). Repeat these steps for the other three combinations of slit width and slit separation
until you have studied all four combinations on the wheel.
Diffraction of Light
Theory: Light that is incident upon a
more slit will be diffracted and produce a
diffraction pattern on a distant screen. If
light with wavelength  is incident upon a
slit of width a, the light will interfere with
itself destructively and produce a series of
maxima and minima on a distant screen;
minima are produced when
a sin  = m 
(see Figure 4) where m is an integer (m= 1,
2, ...), and the angle  is given by
tan  = y/D
where D is the distance between the slit
and the screen, and y is the distance along
the diffraction pattern (see Figure 4).
(Note, in the plots you will produce for this
lab, the intensity will be on the y-axis, and
the distance along the diffraction pattern,
y, will be on the x-axis.)
In this lab we will study the diffraction of
light that passes through slits with
different properties.
Figure 4 Diagram of the light path (a) and path
difference (b) for a single slit diffraction pattern
Figure 5 The relative intensity pattern for single
slit diffraction.
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Procedure and Calculations:
1). Remove the Multiple Slit Set wheel, and adjust the laser so it is incident in the
middle of the slit in front of the Light Sensor, if necessary. Do this using the two knobs
on the back of the laser (only make adjustments using the knobs labeled "vertical" and
"horizontal"). Replace the Multiple Slit Set wheel with the Diffraction Set wheel. The
Data Studio settings are unchanged from the previous experiment.
2). Before turning on the laser, check the settings for the the slit width for the slit just
in front of the light sensor. It should be set to setting number 3. Be sure the gain on
the Light Sensor is set to 100. Select the first of the single slits that you are going to
study. Make sure that the diffraction pattern is bright and clear on the white screen. If
it is not, adjust the wheel so the pattern is bright and clear. The laser should only be
on when it is necessary. Otherwise, turn it off. Do not look at the laser light
directly.
3). Before you begin to record data, rotate the wheel around to see the different types of
interference and diffraction patterns produced by the different slit sets on the wheel.
First, we will study the diffraction pattern of the 4 single slits on the wheel.
4). You are ready to record data. Set the wheel on the first of the four double slit sets
you are going to study. Press the Start button. Very slowly, move the wheel behind
the Light Sensor to move the diffraction pattern across the Light Sensor. When you are
done, press the Stop button.
5). Click on "Scale to Fit," which is the icon on the graph in the upper left hand corner.
You should see the intensity pattern of the diffraction pattern.
6). Use the "Smart Tool" (which is the icon that looks like a coordinate graph) to
accurately record the coordinates of the minima of the pattern. Print out the graph.
7). Record the wavelength of the light, the distance between the slit and the screen, and
the width of the slit printed on the Diffraction Set wheel.
8). Repeat these steps for the three other slit widths until you have studied all 4 single
slits on the wheel.
9). Use your observations to determine the width of each slit. For each slit, compare
your experimentally determined width with that printed on the Single Slit Set wheel.
Discuss how the slit width a affects diffraction pattern.
10). Study the diffraction patterns of the square, hexagonal, random opaque dot,
random hole, opaque line and circular aperture patterns.
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