The Michelson Interferometer
The interferometer is a great way to show the wave nature of light.
Our goal was to design an interferometer that would be stable enough that you could pull it off the shelf, turn it on and display an interference pattern in just a few seconds. We chose the Michelson version of the interferometer because it is conceptually easy to understand. It is easy to set up and provides a lot of flexibility in its use.
The interferometer has a 1/4" thick 10" x 10" aluminum base plate. The interferometer comes with everything you need to make a functional interferometer including a 650 nm laser pointer, two mirror assemblies (one mounted on a piezo disk), beam splitter and expansion lens. All you need to provide is a a screen to display the diffraction pattern. A small white card board box or index card works well. If you want to record a video of the fringes, you will need a web camera. We provide a post and clip to mount the camera. The interferometer is simple to assemble. You only need common tools: typically a no. 1 phillips screw driver and an allen wrench for 10-32 allen head screws. Assembly instructions are provided and can also be found here.
"Here's a picture of a typical interference pattern obtained with our interferometer. The distance bewteen the bright ridges is one wavelength of the laser pointers light. In this case it is 650 nm.
As one of the mirrors is moved, the length of one of the legs changes. This will cause the ridges in the diffraction pattern to move. By tracking the distance one of the ridges moves one can determine the distance the mirror has moved. But just showing a diffraction pattern by itself doesn't create much student excitement. It would be much more useful if you could actually measure something with the apparatus.
About the time I started making the interferometer I met Ken Cecire at an AAPT meeting. He mentioned it would be really neat if I could make an interferometer that one could simulate the effects of a gravitational wave - sort of a mini "LIGO" experiment. In the LIGO interferometer the distance between the mirrors changes as gravitational waves pass through the interferometer.
To simulate a gravity wave I mounted one of the mirrors on a piezo disk. By varying the voltage one can move the disk in and out and actually measure the displacement of the piezo disk by observing the motion of the fringes. If you are industrious you could program a voltage signal to simulate a gravitational wave. Or you could just measure the voltage on the disk as you vary the voltage and come up with a plot of disk displacement versus voltage.
Or you can remove one of the mirrors, and attach a mirror to a nearby wall. Place the apparatus on a support
independent of the wall but near the wall to minimize the distance from the interferometer to the wall. Adjust the mirror on the interferometer to get a diffraction pattern. Then push on the wall and the fringes will move as the wall moves. What a great way to introduce a discussion of rigidity and compressive strength.
Regardless of how you use it, the interferometer allows you to actually measure something rather than just talk about what it can do.
The interferometer has been used in several workshops at meetings of the AAPT and has been universally acclaimed by workshop participants. It has withstood the test of teachers and students, and is now available for purchase. Click here to purchase the Interferometer.