Michelson Morley experiment Introduction The MichelsonMorley experiment was

Michelson Morley experiment

Introduction: The Michelson–Morley experiment was performed in 1887 by Albert Michelson and Edward Morley at Western Reserve University in Cleveland. It attempted to detect the relative motion of matter through the stationary luminiferous ether. The negative results are generally considered to be the first strong evidence against the ether theory. It eventually led to special theory of relativity, in which the stationary ether concept has no role.

ETHER AS A HYPOTHETICAL MEDIUM It was assumed that just as surface water waves require a "medium” to move across (i. e. water), and audible sound requires air or water as medium for propagation, so light waves must also require a medium. So, ‘Luminiferous Ether’ was assumed as a hypothetical medium for the purpose. Because light can travel through vacuum, it was assumed that even vacuum must be filled with ether. The ether was assumed to have a highly unusual combination of properties since the speed of light is very high and material bodies pass through the ether without obvious friction or drag.

Hence scientists assumed ether to be an absolute medium i. e. the only medium in the universe at rest. Hence if it could be detected, the absolute position and absolute motion of any particle in the universe will be known.

Experimental Set up l l It consists of a light source S from which light falls on the glass plate P through a convex lens L. The glass plate is placed at an angle of 45°. It is partially silvered (at back surface). Two mirrors M 1 and M 2 are placed at equal distance L from the glass plate. A telescope T is used to observe the interfering beams.

Theory and working: The incident beam B is partially reflected (beam B 2 ) and partially refracted (beam B 1 ) from silvered surface of glass plate. The beams B 1 and B 2 suffer reflections from mirrors M 1 & M 2 respectively. Beam B 2 travels twice through the glass plate P thereby introducing an extra path difference between interfering beams B 1 and B 2. In order to compensate this extra path difference between both the interfering beams, a glass plate of same material and same thickness is introduced in the path of beam B 1. This plate is called compensating plate.

If T 1 is the time taken by light to travel from glass plate P to mirror M 1 and back, then: T 1= L/(c-v) +L/(c+v) = 2 L/c(1+v 2/c 2) If Tt is the time taken by light from P to mirror M 2 and back, then: Tt =2 L/c(√ 1 -v 2/c 2) =2 L/c(1+1/2. v 2/c 2) ∆t= T 1 - Tt = 2 L/c*1/2. v 2/c 2 = Lv 2 /c 3 The effective path difference between the two beams is given by: Lv 2/c 2 On rotating the interferometer through 90° the path difference between the interfering beams becomes 2 Lv 2/c 2 Hence a fringe shift should take place in the field of view. But actually, no fringe shift could be observed.

Explanation of negative results: Moving earth dragged a little ether with itself. So there was no relative motion between the ether and earth and hence no shift. l Since the earth moves round the sun in its orbit in different directions, it was argued that there is no relative motion between the two and hence no shift. l Lorentz Fitzgerald hypothesis: It was suggested that the dimensions of all bodies moving through ether with velocity v get contracted by a factor √(1 -v 2/c 2). Hence it makes times T 1 and Tt equal in the experiment and hence no shift. l

Conclusions: l Ether has no observable properties. So there is not an absolute space or a fixed frame of reference with respect to which absolute motion of the bodies can be determined. l The velocity of light is same in all directions and does not depend on the motion of the source or the observer or both.

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