Michelson Interferometers  

An interferometer is an instrument for making precise optical measurements. It splits light into two or more beams that travel unequal paths and interfere with each other when reunited. This interference appears as a pattern of light and dark bands called interference fringes. Information derived from fringe measurements is used for precise wavelength determinations, measurement of very small distances and thicknesses, and the study of spectral lines.

Michelson Interferometer

The figure shows a simple Michelson inteferometer that uses a beamsplitter to divide a beam of light into two. A perfect beamsplitter reflects 50% of the incident light and transmits 50%. One beam travels towards a stationary mirror, the other to a mirror that can be moved back and forth. When the beams are recombined at the beamsplitter the light has traveled different distances.

Imaging Michelson Interferometer

An Imaging Michelson interferometer is simply a Michelson interferometer where the telescope focal plane is imaged onto a detector array. An interferogram is recorded for every pixel in the field of view, and hence a spectrum can be obtained for every object.

Four-Port Interferometer

In astronomy, interferometers are used to measure the angular separation between stars, the diameters of stars, and their spectra. It is this last application that we use for NGST.

One of the disadvantages of a classical Michelson is that the beamsplitter reflects 50% of the light back to the source. For IFIRS we have chosen a four-port design, shown here, that wastes none of the light. Instead of flat mirrors, IFIRS uses cube-corner mirrors, which introduce a displacement between the input and output beams.

Famous Michelson Interferometers

One of the most famous applications of a Michelson interferometer is in the Michelson-Morley experiment, an attempt to detect the velocity of the Earth with respect to the hypothetical luminiferous ether, a medium in space proposed to carry light waves. The procedure depended on a Michelson interferometer, because this device can be used to compare the optical path lengths for light moving in two mutually perpendicular directions. Michelson reasoned that, if the speed of light were constant with respect to the proposed ether through which the Earth was moving, that motion could be detected by comparing the speed of light in the direction of the Earth's motion and the speed of light at right angles to the Earth's motion. No difference was found. This null result seriously discredited the ether theories and ultimately led to the proposal by Albert Einstein in 1905 that the speed of light is a universal constant.

More recently the Far Infrared Absolute Spectrophotometer (FIRAS) on the COBE satellite was used to make the most precise measurement of the spectrum of the cosmic microwave background.

According to these measurements, the temperature of the Universe is 2.726 +/- 0.004 K.  The spectrum of the cosmic microwave background is extremely close to that of a perfect black body. The agreement is so good that constraints can be placed that no more than 0.03% of the microwave energy in the Universe could have been emitted more than a year after the Big Bang. Some cosmologists have proposed that such energy might have been released by supermassive supernovas, by black holes, or by the decay of unstable exotic particles.