Michelson Interferometer

Michelson interferometer: In Michelson interferometer the two coherent sources are derived from the principle of division of amplitude. The parallel light rays from a monochromatic source are incident on beams splitter (glass plate) G1 which is semi silvered on its back surface and mounted at 45° to the axis. Light ray incident ‘O' is refracted into the glass plate and reaches point A , where where it is partially reflected (ray 1) and partially transmitted ray 2. These rays then fall normally on mirrors M1 (movable) and M2 (fixed) and are reflected back. These reflected rays reunite at point A again and follow path AT. Since these two rays are derived from same source(at A) and are therefore coherent, can interfere and form interference pattern.

In this geometry, the reflected ray 1, travels an extra optical path, a compensating plate G2 of same thickness as plate G1 ) is inserted in the path of ray 2 such that G2 is parallel to G1 . This introduces the same optical path in glass medium for ray 2 as ray 1 travels in plate G1 (therefore is called a compensating plate). Any optical path difference between the ray 1 and ray 2 is now equal to actual path difference between them.

To understand, how the fringes are formed, refer to fig. An observer at 'T' will see the images of mirror M2 and source S ( M'2 and S' respectively) through beam splitter along with the mirror M1. S1 and S2 are the images of source in mirrors M1 and M2 respectively. The position of these elements in figure depend upon their relative distances from point A .

Fig.A: Michelson Inferometer (Experimental Set-up)

Fig. B : Formation of Fringes

Light from a point (say P ) from extended source appears to come from corresponding coherent points P1 and P2 on S1 and S2 .


If ‘d ' is the separation between mirrors M1 and M2' then ‘2d' is the separation between virtual sources S1 and S2 The path difference between the two parallel rays coming from point P1 and P2 respectively and reaching the eyepiece is equal to .