JEE Main 2025 Apr 7 Shift 1, Physics Q10: Thin Lens and Mirrors

JEE Main2025Apr 7, Shift 1Physics

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

Solution

For two thin lenses separated by distance $d$ , the combined focal length obeys

$\frac{1}{f _{eq}} = \frac{1}{f _{1}} + \frac{1}{f _{2}} - \frac{d}{f _{1} f _{2}}$

Convert to SI: $f_{1} = 0.30$ m, $f_{2} = 0.10$ m, $d = 0.10$ m.

$\frac{1}{f _{eq}} = \frac{1}{0.30} + \frac{1}{0.10} - \frac{0.10}{( 0.30 ) ( 0.10 )}$

$= \frac{10}{3} + 10 - \frac{10}{3} = 10 m^{- 1}$

Power $P = \frac{1}{f _{eq}} = 10$ D.

$∴$ Option (4).

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