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MHT CET 2019 (2 May, Shift 2) · Combination of Lenses

Combination of Lenses question from MHT CET 2019 (2 May, Shift 2)

Q51MHT CET 2019 · 2 MAY · SHIFT 2Easy

Combination of Lenses

A convex lens of focal length ' $f$ ' is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is

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Answer: (b)

For lenses in contact $\frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2}$ .
Convex: $f_1 = +f$ , concave: $f_2 = -f$ , so $\frac{1}{F} = \frac{1}{f} - \frac{1}{f} = 0$ .
$F = \infty$ (the pair acts like a plane glass plate).
So the answer is (b) infinity.

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