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Focus and focal length - class-XII
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The radii of curvature of the surfaces of a double convex lens are 20 cm and 40 cm respectively, and its focal length is 20 cm. What is the refractive index of the material of the lens.?
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A
$\dfrac{5}{3}$
💡 Explanation:
Here $R _1$ = 20 cm, $R _2$ = -40 cm, f = 20 cm
Using lens maker's formula we get,
$\dfrac{1}{20} , = , (\mu , - , 1) \left ( \dfrac{1}{20} , + , \dfrac{1}{40} \right )$
$\dfrac{1}{20} , = , (\mu , - , 1) \dfrac{3}{40} , \Rightarrow , \mu , = , \dfrac{5}{3}$