Name: ___________________________
Group Members: ___________________________
___________________________
___________________________
Interference
Purpose and Objectives
The type of interference we will be looking at is the interference of coherent light. Think of the light
beams in the following discussion as a single continuous wave, such as a sine wave. This is a good
representation of the electric field of the laser light we will be using. Remember, these waves obey the
principle of superposition.
In today’s laboratory experiment you will find the wavelengths of three different colored lasers by
analyzing interference patterns. We will examine the effects of a laser passing through a Single-Slit
aperture and a Double-Slit aperture and the wave nature of light. Your learning objectives for this lab
are as follows:
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
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To explore interference using a Single-Slit aperture and a Double-Slit aperture.
To accurately measure wavelengths of light.
To verify the thin lens equation
Materials
Lasers of differing wavelengths
Double-Slit aperture
Large Sheet of Paper
Ring Stand
Single-Slit aperture
Clip
Modeling Clay
Clamps
Preliminary Questions
1. Read about the principle of superposition in your textbook. Part A: Look at the two waves above
and below one another. Add them together and draw the resultant wave below them. Do the
same thing for Part B.
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2. What type of interference is this called?
Answer for Part A: __________________
Answer for Part B: __________________
3. In Young’s Double-slit experiment, a pattern of light and dark fringes are observed. Light
comes from both slits and causes a light or dark fringe. What is different about the two
sources of light that causes this phenomenon?
4. In your own words, define Diffraction.
5. In your own words, define wavelength. Draw a small wave and indicate the wavelength
Background Information
The Single Slit
When a beam of laser light is shined through a single thin slit, interference is observed on a
projection screen as in the following picture.
Don’t expect to see a pattern like this by shining a flashlight through a slit in a piece of paper. The
light must be of a single continuous wavelength, such as laser light and the size of the slit must be
close to the same size of the wavelength of the light.
In the diagram below, imagine light coming in from the left and passing through a thin slit of width
a. The Intensity of light on a screen a distance D away is represented by the squiggly line. Compare
with the above picture. Two rays of light, one from each side of the slit are shown pointing to a spot
on the screen that is a distance y from the center.
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But you say “Wait! The light is coming from the left, how can it point up to that spot at y?”
Remember, that this is a wave phenomenon. Picture water waves from the left hitting a retaining wall
that has a break in it. Just as the water waves spread out once they pass the break, so do light waves!
Back to the diagram, since the beams of light come from a different side of the slit, they travel a
different distance to the point at y. The electric field sine waves in each beam of light obey the
principle of superposition, so the can add up to a bright spot, a dark spot, or something in between.
Where the dark spots occur can be found using the following relationship:
where λ is the wavelength of the incoming light. θ and a are as shown in the diagram above. From the
geometry of the triangles and for small angles of θ, sinθ ≈ y/D. Using this, the equation becomes:
Example of a single slit
We shine a Helium-Neon laser through a slit that has a width of a = 0.01cm. On a screen that is 6
meters away we get the interference pattern below. We measure the distance from the center
maximum to the second dark spot to be y = 7.6 cm. What is the wavelength of light we are using?
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TRY IT! Plug the numbers into the single slit equation. Remember to use standard units. Your
answer should be 633nm. NOTE: If we used a different color laser light, you would get a different y
value for the second dark fringe!
The Double Slit
What if we shine the light through two slits placed very close together? Each slit has its own single-slit
interference pattern as described above, but in addition, they interfere with each other!
In a similar way to finding the single slit equation for dark fringes we can find an equation for the
double slit pattern. NOTE that the double slit equation below may look the same as the single slit
equation, but this one is for light fringes. It is for the bright spots!
The y in the Double-Slit formula stated above is the distance from the central maximum to the bright
spot at m = 0, 1, 2…. The bright spots would be evenly spaced if not for the superposition of the
single-slit interference pattern over it, as shown below.
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Procedure
1. You will need to select a laser (Red, Green, or Blue) and a rectangular glass piece that has the
slits in it.
2. Adjust the position of the rectangular glass piece that has the slits in so the laser beam hits
the glass surface at 90° and hits the smallest Single-Slit hole. You should see a sharp pattern
on the far wall.
3. Tape a clean sheet of paper to the wall so the light pattern hits it near the top. You need to
make room to trace all three patterns on this single sheet of paper.
4. Using the metric measuring tape, measure the distance between the slits and your paper
‘screen’.
5. Trace the single slit pattern on your paper. Remember, for this one you will be measuring the
distance between the center of the pattern and the dark spots.
6. Repeat steps 2 – 5 for a larger Single-Slit hole.
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7. Now adjust the rectangular glass piece so the laser runs through the smallest Double-Slits.
Move your paper screen accordingly and trace the pattern on your paper, remembering that
you will be measuring the distance between the center of the pattern and the bright spots.
8. Repeat step 7 for one of the larger Double-Slits.
9. Now adjust the rectangular glass piece so the laser runs through the Grating. Move your
paper screen accordingly. Mark the bright Red circles.
10. Repeat steps 1- 9 for each of the different colored lasers.
11. Measure and record the slit widths and separations for the Single-slit and the Double-Slit
rectangular glass pieces.
Analysis
You will now measure the spacing for the dark spots (single-slit) or light spots (double-slit and
grating). From these you will calculate the wavelength of the laser light using the appropriate formula.
These will then be compared to the known wavelengths given earlier.
1. To calculate the wavelength of light from the single-slit pattern we will use m = 1 and the
picture below. However, just taking one measurement will not give a good value for y.
Therefore, we will get an average value for y by dividing the length L by the number of
segments. (In this picture there are 6 segments, it may be different for you)
2. Now, using your pictures and the formula for Dark fringes of a Single Slit, find the
wavelength of light used in your single-slit experiments. Compare this to the accepted value.
3. Repeat what you did in steps 1 & 2 but now for your double-slit pattern. Two things you
should remember are that the double slit formula is for the center of the bright fringes. Also,
since the single-slit pattern is superimposed on the double-slit pattern, you may want to find
an average value of y by just looking at the central light spots.
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