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JEE Main 2026 · 5 Apr · Shift 1, Physics question 42

A compound microscope is designed with two symmetric biconvex lenses. The objective lens is cut vertically, creating two identical plano-convex lenses. One of them is used in place of original objective lens. To retain same magnification keeping the object distance unchanged, the tube length has to be

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Answer and worked solution

Official answer: A

Focal length. A symmetric biconvex lens (radii R) has 1f=2(μ−1)R\dfrac1f=\dfrac{2(\mu-1)}R. Each plano-convex half has 1f′=μ−1R\dfrac1{f'}=\dfrac{\mu-1}R, so f′=2fof'=2f_o.

Magnification. m≈Lfo⋅Dfem\approx\dfrac{L}{f_o}\cdot\dfrac D{f_e}. Doubling fof_o means the tube length L must be doubled to keep m.

(A)

Topics:Optical instrumentsThin lenses and lens maker's formula
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