NEET 2023 — Physics solution

NEET2023Physics

Two thin lenses are of same focal lengths ( $f$ ), 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
$f /4$
B
$f /2$
C
Infinite
D
Zero

Solution

The convex lens has focal length $+ f$ and the concave $- f$ .

$\frac{1}{f _{e q}} = \frac{1}{f} + \frac{1}{- f} = 0 \Rightarrow f_{e q} = \infty$ .

The combination has zero power and acts like a plane slab.

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