Calculate the focal length of the thin lens shown in figure. The points $C_{1}$ and $C_{2}$ denote the centres of curvature. Refractive index of lens is $1.5$ .

$20 c m$

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$40 c m$

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$60 c m$

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$80 c m$

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Solution

The correct option is B
$40 c m$

Given:
Refractive index, $μ = 1.5$
Radius of curvature of first surface, $R_{1} = 10 c m$
Radius of curvature of second surface, $R_{2} = 20 c m$
This is because the first surface is more curved and due to sign convention.

The focal length is given by:
$\frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$
$\frac{1}{f} = (1.5 - 1) (\frac{1}{10} - \frac{1}{20})$
$f = 40 c m$

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Calculate the focal length of the thin lens shown in figure. The points C1 and C2 denote the centres of curvature. Refractive index of lens is 1.5.

Calculate the focal length of the thin lens shown in figure. The points $C_{1}$ and $C_{2}$ denote the centres of curvature. Refractive index of lens is $1.5$ .