A type of interferometer in which monochromatic light is passed through a pair of parallel half-silvered glass plates producing circular interference fringes. One of the glass plates is adjustable, enabling the separation of the plates to be varied. The wavelength of the light can be determined by observing the fringes while adjusting the separation. This type of instrument is used in spectroscopy. The Fabry–Pérot interferometer was invented by Charles Fabry (1867–1945) and Jean Baptiste Alfred Pérot (1863–1925) in 1897.