A double convex lens is made of glass of refractive index $1.5 .$ If its focal length is $30 c m,$ then radius of curvature of each of its curved surface is

30 c m

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15 c m

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18 c m

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None of these

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Solution

The correct option is B $30 c m$
Given, a double convex lens is made of glass where refractive index, $μ = 1.5$ and focal length, $f = 30 c m$ .
As it is double convex lens, let, $R_{1} = + R$ and $R_{2} = - R$ .
Now, according to lens maker's formula,
$\frac{1}{F} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$
Using the given values, we have,
$\frac{1}{30} = (1.5 - 1) (\frac{1}{R} - \frac{1}{- R})$
Or, $\frac{1}{30} = 0.5 (\frac{2}{R}) = \frac{1}{R}$
Therefore, radius of curvature of each of its curved surface is, $R = 30 c m$
So, correct option is - (A) $30 c m$

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