Interference and Diffraction Activities
Wave interference can be demonstrated using a ripple tank. This is just a tank of water in which
objects vibrate to generate waves. Open the following ripple tank simulation:
http://www.falstad.com/ripple/
Note that you have a vibrating point source generating waves that spread out in a circular direction.
Two Point Sources
Now use the drop down menu at the upper right of the screen to change the Setup to “Two Sources”.
Your wave pattern should show constructive and destructive interference as shown below.
D
C
B
A
Note that all points along line A are equidistant from the two point sources.
For points along line B, what is the difference in distance to the point sources in terms of the
wavelength ?
What is the path difference for points along line C?
What is the path difference for points along line D?
Prediction: How do you think the angles between the interference maxima will change if you increase
the separation of the point sources?
Check: You can use your mouse to move the point sources. Change their separation and see if your
prediction is correct.
Prediction: How do you think the angular spread will change if you increase the wavelength?
Check: Use the Source Frequency slider to check your prediction. (Note that increasing the frequency
decreases the wavelength.)
1
Prediction: Now, how do you think the wave pattern will change if you make the points oscillate 180o
out of phase?
Check: Check by moving the slider on the Phase Difference control (lower right) all the way to the
right.
For large distances from the sources compared to their separation, the angular positions of the
interference maxima are given by
m = d sin,
where d is the separation of the point sources and m = 0, 1, 2, ... Explain how this formula is
consistent with the simulation.
Double Slit
Change the setup from Two Sources to “Double Slit”. Describe what you observe. Is this similar to
what you observed for Two Sources?
Single Slit
Now change the setup to “Single Slit”. Again, describe what you observe. How is this similar to the
Double Slit pattern? How is it different?
Single Slit Intensity Distribution
Run this simulation to see how the intensity of light through a single slit varies with position when
projected onto a screen.
http://lectureonline.cl.msu.edu/~mmp/kap27/Gary-Diffraction/app.htm
The positions of the minima (not maxima as for the double slit) are given by
m = a sin,
where a is the slit width and m = 1, 2, ... (not m = 0).
2
Set the slit width to a = 10 µm and the wavelength to  = 500 nm. What is the angular position of the
first minimum?
 = ________ deg
Use the position of the first minimum shown in the intensity curve to calculate the distance from the
slit to the screen.
L = ________ m
Double Slit Intensity Distribution
Now run this simulation to see the intensity curve for light projected through a double slit onto a
screen.
http://lectureonline.cl.msu.edu/~mmp/kap27/Gary-TwoSlit/app.htm
Set the slit separation to d = 30 µm, the slit width to a = 1 µm, and the wavelength to  = 600 nm. (By
making a << d, the two slits are effectively point sources.) Measure the angular position of the first
maximum.
 = ________ deg
Now use the position of the first maximum from the intensity curve to calculate the distance to the
screen.
L = ________ m
3