NEET-XII-Physics
21: Speed of Light
- #1In an experiment to measure the speed of light by Fizeau’s apparatus, following data are used: Distance between the mirrors = 12.0 km,
Number of teeth in the wheel = 180.
Find the minimum angular speed of the wheel for which the image is not seen.Ans : Distance between the mirrors (D) = 12.0 km = 12 × 103 m
Number of teeth in the wheel (n) = 180
Now we apply Fizeau's apparatus
Speed of light, c = 3 × 108 m/s
`` \,\mathrm{\,We\,}\,\mathrm{\,know\,},c=\frac{2Dn\omega }{\,\mathrm{\,\pi \,}}``
`` \Rightarrow \omega =\frac{c\pi }{2Dn}\,\mathrm{\,red\,}/\,\mathrm{\,s\,}``
`` =\frac{\pi c}{2\,\mathrm{\,D\,}n}\times \frac{180}{\pi }\,\mathrm{\,deg\,}/\,\mathrm{\,s\,}``
`` \omega =\frac{3\times {10}^{8}}{24\times {10}^{3}}``
`` =1.25\times {10}^{4}\,\mathrm{\,deg\,}/\,\mathrm{\,sec\,}``
Hence, the required angular speed of the wheel for which the image is not seen is `` 1.25\times {10}^{4}\,\mathrm{\,deg\,}/\,\mathrm{\,sec\,}``
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