Michelson interferometer

Michelson interferometer

Michelson interferometer

    This is one of the oldest types of Interferometers.

    However, Michelson using this Interferometer, established exact relationship between meter and red wavelengths of cadmium lamp.

    The basic Michelson Interferometer consists of a monochromatic light source, a beam splitter and two mirrors.

    It relies on the principle of constructive and destructive interference as one mirror remains fixed and the other is moved.

    It uses a monochromatic light (single wavelength) from an extended source.

    This light falls on the beam splitter (which is a plain parallel plate having a semi-transparent layer of silver at its back) which splits the light into two rays of equal intensity at right angles.

    One ray is transmitted to Mirror M1 and other is reflected through beam splitter to Mirror M2.

    From both these mirrors, the rays are reflected back and these reunite at the semi-reflecting surface from where they are transmitted to the eye of the observer.

    Mirror M2 is fixed and the reflected ray from M1 serves as a reference beam. Mirror M1 is movable. i.e. it is attached to the object whose dimension is to be measured.

    If both the mirrors are at same distance from beam splitter, then light will arrive in phase and observer will see bright spot due to constructive interference.

    If movable mirror shifts by quarter wavelength, then beam will return to observer 180̊ out of phase and darkness will be observed due to destructive interference.

    Each half wavelength of mirror travel produces a change in the measured optical path of one wavelength and the reflected beam from the moving mirror shifts through 360° phase change.

    When the reference beam reflected from the fixed mirror and the beam reflected from the moving mirror re-join at the beam splitter, they alternately reinforce and cancel each other as the mirror moves.

    Thus each cycle of intensity at the eye represents λ/2 of mirror travel.

    It may be noted that when monochromatic light source is used then fringes can be seen over a range of path difference that may vary from a few to a million wavelengths, depending on the source.

    However, when white light is used, then fringes can be seen only if both ray paths are exactly equal to a few wavelengths in total length in glass and air.

    The lengths themselves are not important, but only their differences affect fringe formation.

    So when white light source is used then a compensator plate is introduced in the path of mirror M1 so that exactly the same amount of glass is introduced in each of the paths. (In the path of mirror M2, the glass was coming due to rays passing through beam splitter back surface).

    The various sophistications which have undergone to improve the Michelson’s basic apparatus are:

1. Use of laser as the light source, which means that the measurements can be made over longer distances; and also the beam laser compared to other monochromatic sources has exact and pure wavelength thus enabling highly accurate measurements.

2. Mirrors are replaced by cube-corner reflectors (retro-reflectors) which reflect light parallel to its angle of incidence regardless of retroreflector alignment accuracy.

3. Instead of observing the interference phenomenon by eye, photocells are employed which convert light-intensity variations in voltage pulses which are processed by electronic instruments to give the amount and direction of position change.