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>> $Double convex lenses are to be manufactu$

Question

Double convex lenses are to be manufactured from a glass of refractive index 1.55, with both faces of same radius of curvature. What is the radius of curvature required if the focal length is to be 20 cm?

A

11 cm

B

22 cm

C

7 cm

D

6 cm

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Updated on : 2022-09-05

Solution

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Correct option is B)

Lens maker's formula,
$\frac{1}{f} = (μ - 1) (\frac{1}{R _{1}} - \frac{1}{R _{2}})$
Here, f = 20 cm, $μ$ = 1.55, $R_{1}$ = R, $R_{2}$ = -R
$\frac{1}{2 0} = (1.55 - 1) (\frac{1}{R} - \frac{1}{( - R )}) o r \frac{1}{2 0} = 0.55 \times \frac{2}{R}$
$\Rightarrow R = 1.1 \times 20 = 22 c m$

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