question_answer

Diameter of a planoconvex lens of focal length 37 cm is 6 cm. It's thickness at the centre is 5 mm. The speed of light in the material of the lens is -

A) \[{{10}^{8}}m/s\]

B) \[2.4\times {{10}^{8}}m/s\]

C) \[12\times {{10}^{8}}m/s\]      

D) \[3\times \text{1}{{\text{0}}^{\text{8}}}m/s\]

Correct Answer: B

Solution :

\[{{R}^{2}}=9+{{(R=0.5)}^{2}}\]

\[R=9.25\,\,cm\]

\[\frac{1}{37}=(\mu -1)\left( \frac{1}{\infty }+\frac{1}{R} \right)\]

\[\Rightarrow \mu =1.25=\frac{c}{v}\]

\[v=\frac{3\times {{10}^{8}}}{1.25}=2.4\times {{10}^{8}}m/s\]