Physics: Principles and Problems

Chapter 19: Interference and Diffraction

In the News

The laser gyro: a navigational aid that uses the theory of relativity.

April 2005

The laser gyro, also known as a ring gyro, is a device that illustrates the constancy of the speed of light as predicted by the theory of special relativity. It also provides the basis for a very useful navigational instrument.

Caution: Do not eat this doughnut

Consider a large glass doughnut. We split the beam of a laser so that one beam of light shines clockwise around the doughnut, while the other shines in the counterclockwise direction. As the beams move through each other they produce an interference pattern of light and dark nodes around the doughnut. .

The doughnut can rotate, but the interference pattern won't.

Recall that the theory of special relativity tells us that the speed of light is constant regardless of the movement of the medium through which it travels, the light source, and the detector with which we observe the light. So if we rotate the loop (like a steering wheel) the position of the nodes of the interference pattern will remain stationary with respect to the fixed stars. This phenomenon is known as the Sagnac effect.

Tiny, tiny nodes. You need a microscope to see 'em.

If we mounted a powerful system of lenses—a microscope, actually—on our glass doughnut, we would see the nodes in the interference pattern as a line of spots. When we rotate the doughnut—microscope, laser and all—these fixed spots will appear to move past the microscope. By counting the spots as they move past the microscope, we can determine the magnitude, velocity, and direction of the loop's rotation. In actual practice, a sensitive photocell is used with the system of lenses to count the spots as they move by.

The latest versions look like George Washington's cocked hat.

More recent laser gyros send the laser beams along a triangular path that has high-quality mirrors or prisms at each vertex. To determine the rotation of the device, two light detectors are used: one looks at the counterclockwise beam, the other at the clockwise beam. As the triangle rotates, the devices see slightly different frequencies of light due to the Doppler effect. The difference between these frequencies is proportional to the angular velocity of the triangle's rotation. The angular velocity reading is multiplied by the reading of a timer to determine the total angle through which the device has rotated.

Why three?

Three laser gyros mounted in an aircraft, spacecraft, or submarine will indicate the heading and attitude (that is, the direction that the nose is pointing) of the vehicle, thus allowing accurate navigation through sea or space.

Activities:

Show how three gyroscopes would be mounted so that they will determine the attitude and path of an airplane. Assuming that an airplane's direction is known when it takes off, and that the aircraft is level when it does so, how would we know the direction and attitude of the airplane at any time during the flight by just watching the gyros?

What is meant when we say “the interference pattern is stationary with respect to the fixed stars?”

Use a search engine to look up 'inertial guidance system' and learn how it works.

References:

http://www.phys.canterbury.ac.nz/research/ring_laser/ring_open.html

http://www.spie.org/web/oer/september/sep96/gyro.html

http://www.fas.org/man/dod&-101/navy/docs/fun/rlg.htm

http://sm3a.gsfc.nasa.gov/messages/129.html

Glencoe Online Learning CenterScience HomeProduct InfoSite MapContact Us

The McGraw-Hill CompaniesGlencoe