WHAT IS AN INTERFEROMETER?

Picture of Michelson Inteferometer from http://www.3dimagery.com/michelsn.html

A Michelson Interferometer is a device used to measure the speed of light in precise optical measurements. It does this by splitting light into two or more beams that recombine and interfere with each other causing the interference fringes. The interferometer basically consists of a light source, a beamsplitter, and two (or more) mirrors to reflect the light.
Click here to see set up and applications.

The interference pattern for a Michelson interferometer is circular-- that is, it produces concentric circles of light and dark "fringes". When one mirror on the interferometer is moving, the path difference between the two split beams of light changes, and the interference pattern is seen

Interference is a phenomenon that occurs when two photons of light interact with each other in such a way that their waves sum to either increase or decrease their total amplitude. Complete constructive interference occurs when two electromagnetic waves are of the same frequency and in phase; destructive interference occurs when two electromagnetic waves of the same frequency have a phase difference of one-half wavelength. When complete destructive interference occurs, no light can be detected. Similarly, complete constructive interference results in intensity quadrupling (intensity is proportional to the square of amplitude). The following picture demonstrates these effects.

Constructive Interference

Destructive Inteference

Picture from: http://webphysics.davidson.edu/Alumni/DoNeumann/lab.htm/Interfer/contents.htm

WHY WAS IT MADE?

Why? Originally, the inventors of the interferometer produced it to measure the speed of light so they could determine the existence of ether. Since then it has been important in measuring the wavelengths of light, using the wavelengths of light to measure very small distances (up to 0.5 microns), to measure extremely small times (up to 1x10-15 seconds), and to study optical media.

Picture from http://www.3dimatery.com/michelsn.html