The radius of curvature for a convex lens is 40 cm. If its refractive index is 1.5, the focal length will be __________.

40 cm

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20 cm

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80 cm

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60 cm

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Solution

The correct option is A 40 cm
Given:
Radius of curvature $R_{1}, R_{2} = 40 c m$
Refractive index $n = 1.5$
We know by lens maker's formula,

$\frac{1}{f} = (n - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$
$= (1.5 - 1) (\frac{1}{40} - \frac{1}{- 40}) = 0.5 (\frac{1}{20}) = 40 c m$

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