Rayleigh interferometer

The Rayleigh interferometer employs two beams of light from a single source, and determines the difference in optical path length between the two paths using interference between the two beams when they are recombined following traversal of the paths. An example is shown in the figure. Light from a source (top) is collimated by a lens and split into two beams using slits. The beams are sent through two different paths and pass through compensating plates. They are brought to a focus by a second lens (bottom) where an interference pattern is observed to determine the optical path difference in terms of wavelengths of the light.