Q. Explain Michelson Morley Experiment?

The Michelson Morley experiment was an attempt to detect the velocity of earth with respect to the aluminiferous ether, a medium in space proposed to carry light waves. A schematic diagram of the Michelson Morley experiment is shown in Fig. a collimated beam of light from the source S is incident upon a half silvered glass plate P placed at 45 to the beam. It splits into two beams one is reflected and second is refracted. These beam travel right angles to each other. They incident normally on mirrors M1 and M2 and reflected back to P1 are directed towards a telescope T and interference take place. The interference fringes can be observed by the telescope. Let the mirrors M1 and M2 be at the same distance from the half silvered glass plate P1. If the apparatus were at rest, reflected and refracted beam would take the same time to return to P1. Let us assume the earth is moving in space through the ether with a velocity u therefore the apparatus is moving in space with velocity u. Suppose c is the velocity of light through ether. The refracted beam moving towards M2 has velocity relative to the apparatus on the outgoing journey and on the return journey. If T2 is total time by this beam to go from P1 to M2 and back, then the reflected beam travelled towards mirrors M1 will strike the mirror at A1 but only after P1 has moved to a new position P2. The component of velocity of light in the direction perpendicular to the motion of the apparatus is therefore, the time for the round trip P1 A'P2 is the time difference between two paths due to the motion of the instrument relative to the either is given by :
A fringes shift of this amount is readily detected with the apparatus. It should then be possible to measure the fringe shift and form it determine the velocity of the earth relative to the ether. Michelson and Morley could not detect any shift in the fringes when interferometer was rotated through 90. This indicates that the relative velocity between the earth and the ether is zero.