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Lens Maker's Formula

A biconvex le...

Question

A biconvex lens made of glass(refractive index $1.5$ ) has two spherical surfaces having radii $20$ cm and $30$ cm. Calculate its focal length.

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Solution

Refractive index of glass $μ = 1.5$

Given : $R_{1} = 20 c m$ $R_{2} = - 30 c m$

Using lens maker formula :

$\frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$

Or $\frac{1}{f} = (1.5 - 1) (\frac{1}{20} - \frac{1}{(- 30)})$

$⟹ f = 24 c m$

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