JEE Main · 2025 · 07 Apr 2025 · Shift 1 · Geometrical Optics

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Two thin convex lenses of focal lengths 30 cm and 10 cm are placed coaxially, 10 cm apart. The power of this combination is:

(A)5 D
(B)1 D
(C)20 D
(D)10 D✓ Correct

Answer: (D)

Solution

For two thin lenses separated by

d

, the combined power is

P = P_{1} + P_{2} - d P_{1} P_{2} .

With

f_{1} = 0.3

f_{2} = 0.1

m and

d = 0.1

m, the individual powers are

P_{1} = \frac{10}{3}

D and

P_{2} = 10

D, so

P = \frac{10}{3} + 10 - 0.1 \times \frac{10}{3} \times 10 = \frac{10}{3} + 10 - \frac{10}{3} = 10 D .

The power of the combination is

10

JEE Main · 2025 · 07 Apr 2025 · Shift 1 · Single correct option

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