Lummer–Gehrcke interferometer
Encyclopedia
The Lummer–Gehrcke interferometer or Lummer–Gehrcke plate is a multiple-beam interferometer similar to the Fabry–Perot etalon, but using light at a steep angle of incidence. The interferometer consists of a long plate of glass
or quartz
, with faces that are polished accurately flat and parallel. Light bounces back and forth inside the plate, striking the faces at an angle just below the critical angle
as it propagates along. Because of the steep angle of incidence
, nearly all of the light is reflected, but a tiny fraction leaks out on each bounce. As in a Fabry–Perot interferometer, the light that leaks out has phase
that depends on how many times it has bounced inside the plate. A lens is used to overlap light that has emerged after varying numbers of bounces, producing an interference pattern. A key difference from a Fabry–Perot etalon is that input light that reflects off of the surface of the plate does not contribute to the interference.
Lummer–Gehrcke interferometers are now rarely used, having been largely replaced by Fabry–Perot interferometers using modern dielectric reflective coatings
.
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
or quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, with faces that are polished accurately flat and parallel. Light bounces back and forth inside the plate, striking the faces at an angle just below the critical angle
Critical angle
Critical angle can refer to:*Critical angle the angle of incidence above which total internal reflection occurs*Critical angle of attack, in aerodynamics; the angle of attack which produces the maximum lift coefficient...
as it propagates along. Because of the steep angle of incidence
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...
, nearly all of the light is reflected, but a tiny fraction leaks out on each bounce. As in a Fabry–Perot interferometer, the light that leaks out has phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
that depends on how many times it has bounced inside the plate. A lens is used to overlap light that has emerged after varying numbers of bounces, producing an interference pattern. A key difference from a Fabry–Perot etalon is that input light that reflects off of the surface of the plate does not contribute to the interference.
Lummer–Gehrcke interferometers are now rarely used, having been largely replaced by Fabry–Perot interferometers using modern dielectric reflective coatings
Dielectric mirror
A dielectric mirror is a type of a mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified...
.