Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Double-Source Interference


  • ripple tank
  • ruler, protractor, and compass

1. Observe the wave pattern formed by a single source. Try adjusting the frequency at which the motor runs. What do you have to do to the frequency in order to increase the wavelength, and what do you have to do to decrease it?

2. Observe the interference pattern formed by two sources. For convenience, try to get your wavelength as close as possible to 1 cm. We'll call this setup, with λ= 1 cm and d=2.5 cm, the default setup.

3. Imagine that you were to double the wavelength and double the distance between the sources. How would a snapshot of this wave pattern compare with a snapshot of the pattern made by the default setup? Based on this, how do you predict the angles of the maxima and minima will compare?


Test your predictions.

4. On a piece of paper, make a life-size drawing of the two sources in the default setup, and locate the following points:

A. The point that is 10 wavelengths from source #1 and 10 wavelengths from source #2.

B. The point that is 11 wavelengths from #1 and 11 from #2.

C. The point that is 10 wavelengths from #1 and 10.5 from #2.

D. The point that is 11 wavelengths from #1 and 11.5 from #2.

E. The point that is 10 wavelengths from #1 and 11 from #2.

F. The point that is 11 wavelengths from #1 and 12 from #2. You can do this either using a compass or by putting the next page under your paper and tracing.

What do these points correspond to in the real wave pattern?

5. Make a fresh copy of your drawing, showing only point E and the two sources, which form a long, skinny triangle. Now suppose you were to change the default setup by doubling d, while leaving λ the same. Realistically this involves moving one peg over one hole, while leaving the other peg in the same place, but it's easier to understand what's happening on the drawing if you move both sources outward, keeping the center fixed. Based on your drawing, what will happen to the position of point E when you double d? How has the angle of point E changed?


Test your prediction.

6. In the previous part of the exercise, you saw the effect of doubling d while leaving λ the same. Now what do you think would happen to your angles if, starting from the standard setup, you doubled λ while leaving d the same?


Try it.

7. Suppose λ was a millionth of a centimeter, while d was still as in the standard setup. What would happen to the angles? What does this tell you about observing diffraction of light?


Last Update: 2010-11-11