JEE Main · 2026 · 05 Apr 2026 · Shift 2 · Geometrical Optics

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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 $∣ f_{convex} ∣ > ∣ f_{concave} ∣$ ✓ Correct
(B)behaves as concave lens if $∣ f_{convex} ∣ < ∣ f_{concave} ∣$
(C)behaves as convex lens if $∣ f_{convex} ∣ > ∣ f_{concave} ∣$
(D)Focal length of the lens system will change if the position of two lenses are interchanged

Answer: (A)

Solution

For two thin lenses in contact the combined focal length satisfies

\frac{1}{f} = \frac{1}{f _{1}} + \frac{1}{f _{2}}

, which is symmetric and so unchanged if the lenses are interchanged. Taking the convex focal length as positive and the concave as negative,

\frac{1}{f} = \frac{1}{∣ f _{convex} ∣} - \frac{1}{∣ f _{concave} ∣}

. When

∣ f_{convex} ∣ > ∣ f_{concave} ∣

\frac{1}{∣ f _{convex} ∣} < \frac{1}{∣ f _{concave} ∣}

, so

\frac{1}{f} < 0

and the focal length is negative. The combination behaves as a concave lens when

∣ f_{convex} ∣ > ∣ f_{concave} ∣

JEE Main · 2026 · 05 Apr 2026 · Shift 2 · Single correct option

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JEE Main · 2026 · 05 Apr 2026 · Shift 2 · Geometrical Optics

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