Diameter of a plano - convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is 2 × 10 8 m/s, the focal length of the lens is :
Text Solution
Verified by ExpertsThe correct answer is:
C
3 2 + (R – 3mm) 2 = R 2
⇒ 3 2 + R 2 – 2R(3mm) + (3mm) 2 = R 2 ⇒ R ≈ 15 cm
⇒ f = 30cm
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