What Is the Michelson Interferometer?

A Michelson interferometer is a device that splits a beam of light, bounces the two beams off separate mirrors, and recombines them from different paths. Inside the instrument, a moving mirror changes the path of one beam. When the two light beams come together again, they interfere with one another; a detector is included to measure the changes in intensity. The patterns created have been used to study the wave-like properties of light, so these principles can be applied to other measurements. Many two-beam interferometers are based on the Michelson interferometer, which was invented by Albert Abraham Michelson in the early 1890s.

The basic structure of the Michelson interferometer consists of two mirrors that sit perpendicular to one another, and a beamsplitter mounted at a 45° angle to each mirror. One mirror can turn to one side or another. When light enters the device, it hits a beamsplitter that reflects part of the light and transmits another part. Each beam hits a separate mirror. When reflected back, the changes in position of one mirror alter the path of one beam to change the interference effect.

Beam intensity can then be measured by graphing the intensity versus the path difference, on a chart called an interferogram. This early form of interferometer has been used in the development of instruments that can measure radiation at specific ranges in the light spectrum. Fourier Transform Spectroscopy is based on the Michelson interferometer, which is able to create a picture of all the wavelengths in the light sample. The interferometer can also accept more light than other instruments and is more sensitive, especially to infrared light.

A Michelson interferometer can be used to measure the wavelength of specific substances, such as sodium or helium. Its ability to detect gases and various other elements is useful in monitoring the content of the atmosphere. The device is sometimes used by astronomers to measure the size and composition of other planets and stars light years away. For use in space, interferometers can also detect how fluids are affected by convection currents, in order to measure the force of gravity.

Various mathematical formulas are used to interpret the results of a Michelson interferometer. Angles, beam intensity, and light wavelengths need to be understood from a numerical perspective. Proper education and experience helps to understand what the measurements mean and to apply basic principles to the device’s operation, in whichever application it is being used in.