NEET-XII-Physics
21: Speed of Light
- #2In an experiment with Foucault’s apparatus, the various distances used are as follows:
Distance between the rotating and the fixed mirror = 16 m
Distance between the lens and the rotating mirror = 6 m,
Distance between the source and the lens = 2 m.
When the mirror is rotated at a speed of 356 revolutions per second, the image shifts by 0.7 mm. Calculate the speed of light from these data.Ans : Distance between the rotating and the fixed mirror (R) = 16 m
Distance between the lens and the rotating mirror (b) = 6 m
Distance between the source and the lens (a) = 2 m
Mirror is rotated at a speed of 356 revolutions per second
⇒ ω = 356 rev/s= 356 × 2 π rad/sec
Shift in the image (s) = 0.7 m = 0.7 × 103 m
In Foucault experiment, speed of light is given by
`` c=4{\,\mathrm{\,R\,}}^{2}\frac{wa}{s(\,\mathrm{\,R\,}+b)}``
`` =\frac{4\times (16{)}^{2}\times 356\times 2\,\mathrm{\,\pi \,}\times 2}{(0.7)\times {10}^{-3}(16+6)}``
= 2.975 × 108 m/s
Therefore, the required speed of light is 2.975 × 108 m/s.
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