Introduction to interferometery

Schematic of a Michelson interferometer. This is one of the earliest types of interferometer which rely on a beam splitter to separate a single light source along two arms. One arm is of fixed length, referred to here as the reference arm, and the other can move along the optical axis, referred to here as the test arm. The reflected light forms an interference pattern (similar to that shown below) at the detector. In early experiments this would be a screen, modern experiments use highly sensitive photomultipliers.

If the length of the test arm changes the interference pattern changes, i.e. the bright and dark fringes change position. By observing the movement of these fringes very accurate measurements can be made of the displacement of the end of the test arm.