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  4. Double convex lens are to be manufactured from a glass of refractive index 1.55, with both faces of the same radius of curvature. What is the radius of curvature required if the focal length is to be 20 cm ?

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Q. Double convex lens are to be manufactured from a glass of refractive index $1.55,$ with both faces of the same radius of curvature. What is the radius of curvature required if the focal length is to be $20 \,cm ?$

Ray Optics and Optical Instruments

Solution:

$\frac{1}{f}=(\mu-1)\left[\frac{1}{R_{1}}-\frac{1}{R_{2}}\right]$
$\frac{1}{20}=(1.55-1)\left[\frac{1}{R}-\frac{1}{(-R)}\right]$
$\frac{1}{20}=\frac{0.55 \times 2}{R} \Rightarrow R=22\, cm$