A plano-convex lens is made of material of refractive index 1.5. The radius of curvature of curved surface is 20 cm. If its plane surface is silvered, the focal length of the resulting optical system is:
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Question

A plano-convex lens is made of material of refractive index 1.5. The radius of curvature of curved surface is 20 cm. If its plane surface is silvered, the focal length of the resulting optical system is:

Options

G1 Physics · Accepted Answer

Step 1: A plano-convex glass lens has 1/f_lens = (1.5 − 1)/R = 0.5/20, giving f_lens = 40 cm. Step 2: When plane surface is silvered, the ray passes through the lens twice. Equivalent optical power P_eq = 2P_lens. Step 3: Effective focal length f_eq = f_lens/2 = 20 cm. Final answer: (B) 20 cm.