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Michelson Interferometer

Michelson interferometer consists of two highly polished mirrors M₁ and M₂ and two plane glass plates A and C parallel to each other. The rear side of the glass plate A is half silvered so that light coming from the source S is equally reflected and transmitted by it. Light from a monochromatic source S after passing through the lens L, falls on the plate A. The lens L makes the beam parallel. The plate A is inclined at an angle of 45˚. One-half of the energy of the incident beam is reflected by the plate A towards the mirror M₁ and the other half is transmitted towards the mirror M₂. These two beams (reflected and transmitted) travel among two mutually perpendicular paths and are reflected back by the mirrors M₁ and M₂. These two beams return to the plate A. The beam reflected back by M₁ is transmitted through the glass plate A and the beam reflected back by M₂ is reflected by the glass plate A towards the eye. The beam going towards the mirror M₁and reflected back, has to pass twice through the glass plate A. Therefore, to compensate for the path, the plate C is used between the mirror M₂ and A. The light beam going towards the mirror M₂ and reflected back towards A also passes twice through the compensating plate C. therefore, the path of the two rays in glass are the same. The mirror M₁is fixed on a carriage and can be moved with the help of the handle H. The distance through which the mirror M₁is moved can be read on the scale. The planes of the mirrors M₁ and M₂ can be made perfectly perpendicular with the help of the fine screws attached to them. The compensating plate is a necessity for white light fringes but can be dispensed with, while using monochromatic light.

If the mirrors M₁ and M₂ are perfectly perpendicular, the observer’s eye will see the images of the mirrors M₁and M₂ through A. There will be an air film between the two images and the distance can be varied with the help of the handle H. The fringes will be perfectly circular. If the path travelled by the two rays is exactly the same, the field of view will be completely dark. If the two images of M₁ and M₂ are inclined (the mirrors M₁ and M₂ not perfectly perpendicular) the enclosed air film will be wedge-shaped and straight line fringes will be observed. When the mirror M₁ is moved away or towards the glass plate A with the help of handle H, the fringes cross the centre of the field of view of the observer’s eye. If M₁ is moved through a distance λ/2, one fringe will cross the field of view and will move to the position previously occupied by the next fringe.