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in Ray Optics and Optical Instruments by (26.7k points)
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Double-convex lenses 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 of the lens is to be 20 cm?

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\(\frac{\mu_2}{\mu_1}= \mu=1.55\)

R1 = R2 = R

f = 20 cm

\(\frac{1}{f}=(\mu-1)\Big(\frac{1}{R}+\frac{1}{R}\Big)\)

\(\frac{1}{20}\) = (1.55 - 1)\(\frac{2}{R}\)

R = 0.55 × 2 × 20 = 22 cm.

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