Michelson Interferometer

                Mirror
                ---------         /-\
 Beam Splitter     |             /\//
              \    |            |-\\\  Material
               \   |            /-\\\\ Under
 +------+       \  |/           \--//- Test
 |Laser |----------/------------ \-\||
 +------+         /|             /-/|/
                   |            |////
                   |             \\/\
                   |
              +-----------+
              | Detector  |
              |           |
              +-----------+

Collimated laser light is emitted from a source onto a beam splitter. Half the light impinges onto the material being tested. Half the light is directed to a mirror. The light beams from the mirror and material under test recombine at the detector. Constructive and destructive interference between the beams, caused by variations in the pathlength of the light, are observable by differences in the amplitude of the light at the detector. Michelson interferometers are remarkably useful and ubiquitous devices for small-scale measurements of a wide variety of properties.

The Michelson interferometer is an amplitude division interferometer in which rays from a light source hit a beamsplitter. Half the energy of the incident beam is thus reflected in one arm of the interferometer, and the other half is transmitted. At the end of each arm, a mirror reflects the light back towards the beamsplitter. Again, half the light from each arm is transmitted and half of it is reflected, so that in average, half the light is reflected back from the Michelson interferometer towards the source, and half of it goes to the output port where habitually lies a detector.

Depending on the length travelled by the light in each arm (the optical path), light experiences constructive or destructive interference at the detector. If no light reaches the detector because of destructive interference, then the principle of conservation of energy implies that all the light is reflected back at the source. If no light is reflected back at the source, then the condition for constructive interference is met and all the light is transmitted to the detector.

Moving one of the mirrors changes the optical path difference between the two arms and thus the interference condition. Recording the signal from the detector as one changes the optical path difference produces an interferogram. The Fourier transform of the interferogram is the spectrum of the light source. Computing the Fourier transform of interferograms to get spectra is known as Fourier transform spectroscopy or spectrometry.

When a Michelson interferometer accepts divergent light rays (so that it has a non null field of view), a spatial interference pattern appears at the output port. This pattern looks like concentric circles and is commonly called the bull's-eye. These are Haidinger equal inclination fringes and are due to the fact that off-axis rays do not experience the same optical path difference as on-axis rays.