Two thin convex lens of focal lengths 30 cm and 10 cm are placed coaxially, 10 cm apart. The power of this combination is: [2025]

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Solution

Q.
Two thin convex lenses of focal lengths 30 cm and 10 cm are placed coaxially, 10 cm apart. The power of this combination is: [2025]

1 5 D

2 1 D

3 20 D

4 10 D

Ans.
(4)

$f_{1} = 30 cm, f_{2} = 10 cm$

$\frac{1}{f_{eq}} = \frac{1}{f_{1}} + \frac{1}{f_{2}} - \frac{d}{f_{1} f_{2}}$ , d = distance between lens

$\frac{1}{f_{eq}} = \frac{1}{0.3} + \frac{1}{0.1} - \frac{0.1}{(0.3) (0.1)}$

$\frac{1}{f_{eq}} = \frac{1}{0.1}$

Power = $\frac{1}{f_{eq}} = 10 D$

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