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Michelsonís Method for Measuring the Speed of Light
In 1931, Albert Michelson devised a method of measuring the speed of light, directly, by finding how long it took to move a measured distance.
The diagram below, showing the approximate arrangement of the apparatus, is not to scale (but I assume you guessed that!)
 
 
Light from the source passes through a narrow slit.
It is reflected by face A of the octagonal (8 sided) metal prism.
It then travels a distance of a few kilometres and returns to be reflected by face B.
When the prism is stationary, a stationary image of the slit is observed.
The prism is now rotated.
If the prism rotates fast enough, when light returns to the prism, face B is no longer in the right position to reflect it into the observerís eye.
The image of the slit disappears.
However, if the speed of rotation is increased, at a certain speed of rotation, the image of the slit reappears.
This is because the time taken for light to go from face A to face B was the same as the time taken by the prism to rotate 1/8th of a revolution.
So, the calculation needed to find the speed of light is remarkably simple:
If the prism completes n rotations per second then
Therefore, the time, t taken for the light to cover the distance, s is 1/8th of this
and so the speed of light c is given by
In 1931, Michelson found c = 2.99774◊108ms-1
The modern value is c = 2.997925◊108ms-1
 
Although the calculation is trivially simple don't forget the practical difficulties involved in this experiment: for example, measuring the distance moved by the light to a satisfactory level of precision...
 
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