A plano-convex lens is made of a material of refractive index μ = 1.5. The radius of curvature of curved surface of the lens is 20 cm. If its plane surface is silvered, the focal length of the silvered lens will be :
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When a ray falls on convex surface of a plano-convex lens, then it is first refracted and reflected from plane surface finally refracted from convex surface. Thus, two refractions and one reflection take place. So, focal length of plano-convex lens is
= 
....(i)
Here f m = ∞
Now, 
= ( μ – 1) 
⇒ 
= ( μ – 1) 
= 
∴ 
= 
= 
or F = 
....(ii)
Given, R = 20 cm, μ = 1.5
Hence, F = 
= 
= 20 cm
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