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A Michelson Interferometer Simulator

A Classroom Activity

LIGO thanks Diane Fisher at NASA's SpacePlace for the concept on which this activity is based. NASA provides a classroom activity featuring a LISA-type interferometer.

Introduction

Interferometers allow students to watch the constructive and destructive interference of light. However the interference pattern itself is a black box since the eye cannot recognize individual crests of light waves. A model of an interference pattern will give students a way to penetrate the black box by visualizing the formation of interference fringes. In this activity students will use an interferometer simulator to provide such a visualization.

The movie Einstein's Messengers describes LIGO's use of large interferometers as gravitational wave detectors. How will gravitational wave detections occur? As gravitational waves pass the earth, they will put ripples into the space inside LIGO's interferometers. The interferometer mirrors will move as the space vibrates. Laser light in the arms of the interferometer will record the mirror movements in each arm. Finally, the interference pattern formed from the light in both arms will differentiate between the vibrations in the two arms; photodetectors will then read out the fluctuations in the interference pattern. This multi-step sensing chain is a challenge for students to grasp. The interferometer simulator will help students connect the links of the chain.

The educational value of an interferometer simulator increases when used alongside an actual Michelson interferometer. LIGO encourages teachers to consider building an inexpensive interferometer for classroom use. Kit interferometers are also reasonably priced.

Navigate this Web page for the activity by using the following links:

Learning Objectives for the Activity

Students who complete this activity in conjunction with viewing "Einstein's Messengers" will demonstrate the following outcomes.

  • Identify the main components of a Michelson interferometer and their relative positions in the device
  • Draw the paths that light follows through a Michelson interferometer
  • Define "interference pattern" and explain how an interference pattern forms in an interferometer
  • Describe the effect of mirror motion on an interferometer's interference pattern
  • Briefly describe the way that LIGO's interferometers would detect gravitational waves
  • Define "strain" and show how strain would be calculated with interferometer measurements

Connections to Science Standards

Scientific modeling forms an underlying theme of this activity. The simulator illustrates wave interactions that are otherwise invisible. From Benchmarks, p 270: "The usefulness of a model can be tested by comparing its predictions to actual observations in the real world." From Standards, p 176: "Mathematical tools and models guide and improve the posing of questions, gathering data, constructing explanations and communicating results." The content framework of the activity is wave behavior. From Benchmarks, p. 92: "Waves can superimpose on one another ..."

Connections to Science Themes and Concepts

The activity connects to these aspects of physical science:

  • Properties and behavior of waves
  • The use of models in scientific investigations

Connections to Einstein's Messengers

  • 7:20 to 8:45: This portion of the movie closely parallels the simulator activity. Students will see a graphical representation of an interferometer that traces the path of the light through the device. LIGO scientist Rai Weiss discusses the configuration of the detectors that results in the complete destructive interference of light at the photo detectors. The passing of gravitational waves will change this interference condition through the waves' influence on the space between the mirrors and the beam splitter. When using the simulator, students will vary the mirror positions and watch the corresponding effect on the interference pattern.

Guidance for Teachers

Files to download for the activity:

  1. Assembly instructions and materials list for the interferometer simulator
  2. Wave pattern and wave guide templates for the activity
  3. Student worksheet for the activity
  4. Teachers' guide for the activity

Teaching the Activity

The first decision for the teacher to make is the number of simulators to use. We recommend that students work in pairs. Groups larger than three will restrict students' ability to personally manipulate the device. Next the teacher must decide what form of simulator to use, the rapid-assembly temporary model or a more permanent model. Finally the teacher must decide on the complexity of the string path. The assembly instructions illustrate a simple reduced-motion and a more detailed but challenging-to-assemble scheme.

When engaging students in the activity itself, teachers must preapre to deal with questions about manipulating the simulator and about connecting the simulator's behavior to a real interferometer. The passage of Einstein's Messengers referenced above nicely illustrates how light moves through an interferometer. Teachers may wish to repeat this segment of the DVD as students work on the acivity. The American Museum of Natural History LIGO Web site is another valuable resource for this activity.

Additional Gravitational Wave Information

Last modified October 23, 2006
"Einstein's Messenger's" and supporting materials are produced by the National Science Foundation. Any opinions, findings, conclusions or recommendations expressed here are those of the author(s) and do not necessarily reflect the views of the National Science Foundation