An apparatus used to measure very precise lengths, such as the wavelength of light, and to analyse details of spectral lines. It consists of a half-silvered flat mirror which divides a beam of light into two. These beams are returned along the same paths by additional mirrors, then recombined. The interference of light creates dark and light bands or fringes whose positions depend on the lengths of the two light paths. It is named after its inventor, A. A. Michelson.