this is from my notes way back when i did the hsc. hope it helps. (i dont guarantee its usefulness)
[-DESCRIBE & EVALUATE-]
• Michelson-Morley attempt to measure the relative velocity of the Earth through the aether
- Since the Earth was moving relative to the aether, it was thought that there would be an aether wind [think of Earth as a car driving through air].
- The speed of sound in still air isn’t influenced by it. However, if the air is blowing, the speed of sound would vary for an observer. Similarly, the speed of light would have to vary, due to the aether wind.
- The experiment was set-up in a manner that would detect this difference.
- A ray of light was split by a half-silvered mirror. One went through it and one bounced off at 900. After reflecting, the rays came back to the half-silvered mirror and this time again, rays from both mirrors either went through or reflected towards the observing interferometer. The interferometer was sensitive enough to detect the expected interference.
- No interference or discrepancies between the periods of the two split rays was detected.
- The experimental set-up was rotated 900, performed at different heights, yet the ‘null result’ was consistent.
• Role of the Michelson-Morley experiments in making determinations about the wave model of light
- The experiment was based on the aether model and was set out to measure the speed of Earth through the aether. The ‘null result’ showed that the aether model was untrue.
- Experiments were done to validate the hypothesis. Since that failed, this prompted the proposal of an alternative hypothesis.
- Many scientists tried to hold on to the aether model by suggesting that a large object, like Earth, would drag the aether with it. But, these modifications didn’t qualify.
- 20 years after the experiment, Einstein proposed his theory of relativity, which didn’t require the aether model.
- This led the scientists towards to options; try to modify the aether model and prove it or follow Einstein’s theory of relativity.