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NEET PHYSICSEasy

Two thin lenses are of same focal lengths (ff), but one is convex and the other one is concave. When they are placed in contact with each other, the equivalent focal length of the combination will be

A

Zero

B

f4\frac{f}{4}

C

f2\frac{f}{2}

D

Infinite

Step-by-Step Solution

The equivalent focal length FF of two lenses in contact is given by 1F=1f1+1f2\frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2}. For a convex lens f1=ff_1 = f and for a concave lens f2=−ff_2 = -f. Thus, 1F=1f−1f=0\frac{1}{F} = \frac{1}{f} - \frac{1}{f} = 0, which implies F=∞F = \infty.

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