The speed of light is so high that it cannot be measured directly. One method, designed by the American scientist Michelson, uses light travelling between mountain tops tens of km apart.
Light bounces off the octagonal mirror. 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 is
and the time for one eighth of a revolution is
If the distance between the mountains is d m (as in the original experiment) and the prism is rotating at r revolutions per second then
If the prism makes 550 revolutions per second and d=34 km then