A thin convex lens and a thin concave lens are kept in contact and are co-axial. Which of the following statements is correct for this combination of two lenses? (a) behaves as concave lens if |f convex | > |f concave | (b) behaves as concave lens if |f convex | |f concave | (d) Focal length of the lens system will change if the positions of two lenses are interchanged

A thin convex lens and a thin concave lens are kept in contact and… | JEE Main 2026 (5 Apr, Shift 2)

Q1JEE Main 2026 · 5 APR · SHIFT 2Easy

Combination of Lenses

A thin convex lens and a thin concave lens are kept in contact and are co-axial. Which of the following statements is correct for this combination of two lenses?

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Answer: (a)

$\frac{1}{F} = \frac{1}{|f_{\text{convex}}|} - \frac{1}{|f_{\text{concave}}|}$
If $|f_{\text{convex}}| > |f_{\text{concave}}|$ , the second term is larger, so $\frac{1}{F} < 0$ : the system is diverging (concave).
Interchanging thin lenses in contact does not change $\frac{1}{F}$ , so (d) is wrong.
So the answer is (a).

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