Solved: PHYSICS Question for NEET

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A convex lens and a concave lens, each having the same focal length of $25\text{ cm}$ , are put in contact to form a combination of lenses. The power in dioptres of the combination is:

infinite

zero

Step-by-Step Solution

Focal Lengths with Sign Convention: The focal length of a convex lens is positive, so $f_1 = +25\text{ cm} = +0.25\text{ m}$ . The focal length of a concave lens is negative, so $f_2 = -25\text{ cm} = -0.25\text{ m}$ .
Power of Individual Lenses: The power $P$ of a lens in dioptres (D) is the reciprocal of its focal length in meters. Power of convex lens, $P_1 = \frac{1}{+0.25} = +4\text{ D}$ . Power of concave lens, $P_2 = \frac{1}{-0.25} = -4\text{ D}$ .
Power of Combination: When two thin lenses are placed in contact, the equivalent power of the combination is the algebraic sum of their individual powers.

$P_{eq} = P_1 + P_2 = +4\text{ D} + (-4\text{ D}) = 0\text{ D}$ .

Conclusion: The power of the lens combination is zero.

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