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Question

# The radius of curvature of curved surface of a plano-convex lens is 15 cm. The refractive index of the lens material is 1.5. The focal length of plano-convex lens is (surroundings is air)

A
20 cm
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B
40 cm
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C
30 cm
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D
25 cm
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Solution

## The correct option is C 30 cm For a plane surface, R1=∞ For curved surface, R2=−15 cm Also, μ1=1, μ2=1.5 Applying lens maker's formula: 1f=(μ2μ1−1)(1R1−1R2) ⇒ 1f=(1.5−1)(1∞−1−15) ⇒1f=(0.5)(115)=130 ∴f=+30 cm Why this question? Tip: In such questions, first of all direction of incidence must be set to avoid any confusions. Thereafter, apply the lens maker's formula.

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