 In the previous essay we have discussed about relative motion where Mr. A was chasing a car. Mr. A, moving at 3 km/h (w.r.t. reference), found the car’s speed is 7 km/h when the car was moving at 10 km/h with respect to the reference. Light moves at a speed of 3 x 108 m/s hence for a person chasing it at 3 x 108 m/s, light should appear stationary. For example if I turn on a torch and you are move at a speed of 3 x 108 m/s, the light from the torch will not fall on you. But it was found that the speed of light remains constant irrespective of the speed of the observer (or reference).

Albert A. Michelson and Edward W. Morley first performed an experiment in 1887, followed by a series of experiments from 1902-1905 with increasing sensitivity, which proved the aforementioned property of light. Before getting into Michelson-Morley experiment, let’s briefly recap the concept of interference which we have learnt during our H2 Physics tuition classes. Two coherent light waves when interfere in such a way that the trough of one wave falls on the trough of the other and so is the case with crests then it results in constructive interference and it appears like a bright spot. The spot gets dark as difference in phase is introduced, where crest of one wave does not fall on the crest of other (same is the case with troughs). A completely dark spot is observed when the phase difference is such that the crest of one wave falls on the trough of the other and vice-versa. In an interference pattern these dark and bright regions appear in an alternating pattern called as fringes. Two waves that show no phase difference initially can undergo a change in the phase difference due to a difference in the speed at which they travel or the distance they travel.

In an interference experiment, sources that emit coherent light waves with no phase difference initially is chosen. When such light waves emitted from two different locations reach one point, they would have travelled two different distances. This means that they would have travelled for two different time intervals. If the two waves travel for the same time, then the number of oscillations observed in each of the waves is the same and since they don’t have phase difference initially (i.e. crest falls on crest) they would also not have final phase difference. But when these waves travel for two different times, the crest of one wave need not fall on the crest of the other as the number of oscillations is different in each of the cases. This forms the basis of Michelson-Morley experiment.