Question which was troubling physicist throughout 18th century was nature of light. Newton proposed that light was made up of particles called Corpuscle. Since light can travel in vacuum, so light must be a particle. Newton Successfully explained reflection but was not able to explain phenomenon of refraction.
But some his contemporaries like Robert Hook and Christiaan Huygens did not agree with with Newton, and they proposed wave theory of light. They were successful in explaining phenomenon of refraction. But things were not easy. Newtons dominance along with models inability to explain light’s movement in vacuum made this model unsuccessful. So these scientist introduced another element into the model. They gave the theory of ‘Luminiferous aether’.
Existence of aether became popular. It became so popular that it became a topic of classic literature. Arthur conan doyle wrote a book called ‘A Poison Belt’ in which earth has entered in the region of space where a poisonous form of aether exist.
Here is a diagram which showed the setting of experiment.Light from the light source is strikes a beam splitter which is a half silvered mirror. This splits light into 2 separate beam going in 2 different path perpendicular to each other. They are again reflected back such that it retraces its path. These beam recombine and is directed to a screen to produce interference fringes.
This was simple. Now complexity of experiment comes when Michelson and Morley rotated the whole apparatus clockwise.
Let us suppose that aether is present and a photon strikes the beam splitter. The beam splitter divides electron into 2 photons with half of the energy of its parent. Both photon are directed in different direction. Since the whole apparatus is rotating on mercury surface, aether winds are produced. One of the photon will go in direction of it while other will go perpendicular to it*.
Photon going in direction of wind will move faster and one which is moving perpendicular to the wind will be retarded. But when experimentally determined the velocity of both photon was same regardless of fact that it moved in or perpendicular to the wind. This is in accordance to special theory of relativity.
This experiment marked an end to theory of Luminiferous aether. But is light a wave as suggested by Hook or particle as suggested by Newton and Einstein, this is still debatable.
* This is because one of them will be reflected while other one is transmitted through half silvered mirror.