NEET-XII-Physics
21: Speed of Light
Qstn# iv-3 Prvs-Qstn
- #3In 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?Ans : Distance travelled by light between two reflections from the rotating mirror (D) = 4.8 km = 4.8 × 103 m
Number of faces of the mirror, N = 8
Angular speed of the mirror, `` \omega `` = ?
In Michelson experiment, the speed of light (c) is given by
`` c=\frac{\omega DN}{2\,\mathrm{\,\pi \,}}``
where
ω = angular speed
N = number of faces in the polygon mirror
`` \therefore \omega =\frac{2\,\mathrm{\,\pi \,}c}{DN}rad/\,\mathrm{\,s\,}``
`` =\frac{c}{DN}\,\mathrm{\,rev\,}/\,\mathrm{\,s\,}``
`` =\frac{3\times {10}^{8}}{(4.8\times {10}^{3}\times 8)}``
= 7.8 × 103 rev/s
Hence, the required angular speed is 7.8 × 103 rev/s.