Question

In the Foucault's method, the distance from the rotating mirror to concave is $20cm$. Lens is placed mid way between the illuminated slit and the rotating mirror which are $2$ m apart. If the rotating mirror makes $438$ revolutions per second and the deflection of the image is $0.7$ mm calculate the velocity of light

• A. $2.847\times {10}^8$ $m{s}^{-1}$
• B. $2.927\times {10}^8$ $m{s}^{-1}$
• C. $2.649\times {10}^8$ $m{s}^{-1}$
• D. $2.99\times {10}^8$ $m{s}^{-1}$

Answer 1

The correct option is D $2.99\times {10}^8$ $m{s}^{-1}$

$C$ = $\frac{4\omega R^{2}a}{S(R+b)}$

$=\frac{4(2\pi \times 438)\times (20{)}^2\times 1}{0.7\times {10}^{-3}(20+1)}$

$=2.992\times {10}^8$ $m{s}^{-1}$