Solved: PHYSICS Question for NEET

NEET PHYSICSEasy

Two thin lenses are of the 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:

infinite

zero

f/4

f/2

Step-by-Step Solution

Identify Powers: The power ( $P$ ) of a lens is the reciprocal of its focal length ( $P = 1/f$ ).

For the convex lens: $f_1 = +f$ , so $P_1 = +\frac{1}{f}$ .
For the concave lens: $f_2 = -f$ , so $P_2 = -\frac{1}{f}$ .

Combination Formula: For thin lenses in contact, the effective power ( $P_{eq}$ ) is the algebraic sum of individual powers: $P_{eq} = P_1 + P_2$ .
Calculation: $P_{eq} = \frac{1}{f} + \left( -\frac{1}{f} \right) = 0$
Equivalent Focal Length: The equivalent focal length ( $F$ ) is the reciprocal of the effective power: $F = \frac{1}{P_{eq}} = \frac{1}{0} = \infty$
Conclusion: The combination behaves like a plane glass slab with infinite focal length.

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