For the thin convex lens, the radii of curvature are 15 cm and 30 cm respectively. The focal length of the lens is 20 cm. The refractive index of the material is

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Solution

Q.
For the thin convex lens, the radii of curvature are 15 cm and 30 cm respectively. The focal length of the lens is 20 cm. The refractive index of the material is [2024]

1 1.4

2 1.2

3 1.5

4 1.8

Ans.
(3)

$\frac{1}{f} = (\frac{μ_{lens}}{μ_{air}} - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$

$\Rightarrow \frac{1}{20} = (\frac{μ}{1} - 1) (\frac{1}{15} - \frac{1}{(- 30)})$

$\Rightarrow \frac{1}{20} = (μ - 1) (\frac{3}{30})$

$\Rightarrow μ - 1 = \frac{1}{2} \Rightarrow μ = 1 + \frac{1}{2} = \frac{3}{2} = 1.5$

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