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In essence, the Michelson interferometer consists of a beam splitter which divides a beam
of light into two parts of equal intensity, one transmitted, the other reflected. These two
Beams strike mirrors at practically normal incidence and return to the beam-splitter
where they recombine. The basic arrangement is shown in Figure. Light from a source S
is incident on a beam splitter. Half of the beam goes to the mirror M1 and half goes to the
mirror M2. The beams in the Figure 1 recombine and fringes may be seen at detector
position.
Figure 1: Schematic illustration of a Michelson interferometer
Formation of Fringes
A detailed calculation of the nature of the fringes is somewhat complicated, but the
essential features can be deduced from fairly elementary considerations. We first note
that for fringes to be formed it is essential that the two beams be derived from the same
source. This can be understood by noting that monochromatic light can be regarded as a
series of sinusoidal wave-trains which are interrupted at frequent but random intervals as
shown in Figure 2. During each interruption, the sine wave undergoes an abrupt phase
change. The average length of the wave-train between these interruptions (e.g. the
distance l in Figure 2) is called the "coherence length" of the light, and is of order one
meter for ordinary light sources. By a simple extension of this concept, the "coherence
time" is defined as the coherence length divided by the speed of light in the medium. It
follows that the coherence time is usually in the range 10-9 to 10-8 sec. If light from two
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