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  • Diameter of a plano - convex lens is 6 cm and thickness at the centre is 3 mm. If speed of the light in material of lens is 2 x 108 m/s, the focal length of the lens is :

Diameter of a plano - convex lens is 6 cm and thickness at the centre is 3 mm. If speed of the light in material of lens is 2 x 108 m/s, the focal length of the lens is :

 

 

Option: 1

10 cm


Option: 2

15 cm


Option: 3

20 cm


Option: 4

30 cm


Answers (1)

best_answer

\mu=\frac{C}{V}=\frac{3 \times 10^{8}}{2 \times 10^{8}}=1.5

From the fig: R^{2}=(R-3mm)^{2}+ (3 cm)^{2}

Or, \: R^{2}=R^{2}+ 9 mm^{2}-6Rmm +9cm^{2}

R=\frac{9 \times 10^{-4}}{6 \times 10^{-3}}=1.5 \times 10^{-1}m=15 cm

\therefore \frac{1}{f}= (\mu -1)\left( \frac{1}{R}-\frac{1}{\infty} \right )

\frac{1}{f}= \frac{1.5-1}{15cm}\:\:\:or, f=30cm

Posted by

Ritika Jonwal

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