Chapter 27, Problem 3A

How did a spinning octagonal mirror help Michelson calculate the time that light took to make the round trip to the distant mountain?

To determine

The way in which a spinning octagonal mirror help Michelson to calculate the time taken by the light to make the round trip to the distant mountain.

Explanation of Solution

Introduction:

In the Michelson’s experiment, a light source passes through a semi-transparent glass plate and then reflected through a rotating mirror. After that, this reflected light is focused on a concave mirror that returns the light to the rotating mirror and image is formed through a small angle difference.

Firstly, the spherical flat mirror was used in the experiment but Michelson introduced an eight-sided spherical octagonal mirror because this octagonal mirror would need to turn $\frac{1}{8}th$ of a turn in the time in which the light travels to and back from a distant mirror. The calculated value was more accurate than previous experiment.

Conclusion:

Hence, with the help of spinning octagonal mirror, more accurate vale of time taken by the light to make round trip to the distance mountain was obtained.