In a Michelson experiment for measuring speed of light, the distance travelled by light between two reflections from the rotating mirror is 4.8 km. The rotating mirror has a shape of a regular octagon. At what minimum angular speed of the mirror (other than zero) the image is formed at the position where a nonrotating mirror forms it?

Distance travelled by the light wave between two reflections from the rotating mirror (D) = 4.8 km = 4.8 × 103


Number of faces of the mirror, N = 8


The angular speed of the mirror = ω = ?


In the Michelson experiment, the speed of light (c) is given by;



Therefore, we can write ω as,



The angular velocity of the mirror in terms of rev/s




Hence the required angular velocity of the mirror is 7.8 × 103


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