Experimental Measurement of the Speed of Light

The speed of light is so high that very special experiments must be designed to measure it. The experiment shown below, designed by the American scientist Michelson, involves bouncing light between mountains tens of km apart.

The octagonal prism is made to rotate. At most rotation speeds no image of the source will be seen but for certain frequencies the mirror will make an integer number of revolutions plus one eighth in the time light takes to go from face A to face B of the rotating prism and an image of the source will be seen.

The speed of light can then be calculated using the formula

If the prism makes n revolutions per second then the time for one revolution isand the time for one eighth of a revolution isIf the distance between the mountains is 34km (as in the original experiment) and the prism is rotating at 550 revolutions per second then

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