The correct option is (B) – 407.5.
Explanation:
For double convex lens, (1/f) = (n – 1) {(1 / R1) – (1 / R2)}
n = 1.6 , R1 = 15cm, R2 = –15cm
(1/f) = (1.6 – 1) {(1/15) + (1/15)}
= 0.6 × (2/15)
= (1.2 / 15)
∴ f = (15 / 1.2) = 12.5 cm
when lens is in liquid let focal length be f'
∴ (f'/f) = [(a(η)g – 1) / (l(η)g – 1)] .....(1)
Now a(η)g = (ng / na) = 1.6 and l(n)g = (ng / nl) = [(1.6) / (1.63)]
From (1), [(f') / (12.5)] = [(1.6 – 1) / {[(1.6) / (1.63)] – 1}]
= [(0.6 × 1.63) / (– 0.03)]
∴ f' = 12.5 × 1.63 × (– 20) = – 407.5 cm
