Lummer–Gehrcke interferometer
The Lummer–Gehrcke interferometer or Lummer–Gehrcke plate is a multiple-beam interferometer similar to the Fabry–Pérot 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.[1] Light bounces back and forth inside the plate, striking the faces at an angle just below the critical angle as it propagates along.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/Lummer%E2%80%93Gehrcke_interferometer.svg/500px-Lummer%E2%80%93Gehrcke_interferometer.svg.png)
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–Pérot 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–Pérot etalon is that input light that reflects from the surface of the plate does not contribute to the interference.[1]
Lummer–Gehrcke interferometers are now rarely used, having been largely replaced by Fabry–Pérot interferometers using modern dielectric reflective coatings.[1]
See also
- Ernst Gehrcke
- Otto Lummer
References
- ^ a b c Born, Max; Wolf, Emil (1999). "7.6.5 The Lummer–Gehrcke interferometer". Principles of Optics (7th ed.). Cambridge University Press. pp. 380–386. ISBN 0-521-64222-1.
Lummer-Gehrcke interferometer.
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