A convexo-concave diverging lens is made of glass of refractive index 1.5 and focal length 24 cm. Radius of curvature for one surface is double that of the other. Then radii of curvature for the two surfaces are (in cm):

6, 12

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12, 24

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3, 6

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18, 36

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Solution

The correct option is A 6, 12
From LMF, $\frac{1}{f} = (n - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$
Since it is diverging , $f = - 24 c m$
Given that $R_{1} = 2 R_{2}, n = 1.5$
$\frac{1}{- 24} = (1.5 - 1) (\frac{1}{2 R_{2}} - \frac{1}{R_{2}})$
$\frac{1}{- 12} = \frac{- 1}{2 R_{2}}$
$R_{2} = 6 c m, R_{1} = 12 c m$

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