NEET 2022 · PhysicsPrevious Year Question
A biconvex lens has radii of curvature 20 cm each. If the refractive index of the material of the lens is 1.5, the power of the lens is:
Answer: (C) +5 D. Answer: (C) +5 D Solution: Lensmaker's formula: 1/f = (µ − 1)(1/R₁ − 1/R₂).
- A.+2 D
- B.+20 D
- C.+5 D✓
- D.Infinity
Correct Answer
(C) +5 D
Solution & Explanation
Answer: (C) +5 D Solution: Lensmaker's formula: 1/f = (µ − 1)(1/R₁ − 1/R₂). For a biconvex lens: R₁ = +20 cm, R₂ = −20 cm, µ = 1.5. 1/f = (1.5 − 1)(1/20 − 1/(−20)) = 0.5 × (2/20) = 0.5 × (1/10) = 1/20 per cm. So f = 20 cm = 0.20 m. Power P = 1/f(in m) = 1/0.20 = +5 D.
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