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Question
A plano convex lens has diameter of 10 cm and its thickness at the centre is 0.5 cm. Speed of light in the lens is 2 × 108 ms-1. What is the focal length of the lens?
Options
10 cm
17.5 cm
10.5 cm
21 cm
MCQ
Solution
10.5 cm
Explanation:
Consider
∴ (R – 0.5)2 = R2 – 52
R2 – R + 0.25 = R2 – 5
R = 5.25 cm
∴ `1/f = (mu-1)/R`
= `(((3xx10^8)/(2xx10^8)-1)/5.25)`
= `1/10.5`
f = 10.5 cm
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Refraction at Spherical Surfaces and Lenses - Refraction by a Lens
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