The diagram below is not to scale.

Light from the source passes through a narrow slit. It is reflected by face A of the octagonal metal prism. It then travels a distance, s, (a few kilometres) and returns to be reflected by face B.

The prism now rotates. If it rotates fast enough, when light returns to the prism, face B is no longer in the right position to reflect it into the observer’s eye. The image of the slit disappears.

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 1/8^th of a revolution.

If the prism completes n rotations per second then the time for one revolution is 1/n.

Therefore, the time taken for the light to cover the distance, s is given by

So, the speed of light, c is given by

In 1931, Michelson found c = 2·99774×10⁸ms^-1.