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Physics · Geometrical Optics

JEE Main 2026 — 5 April, Morning Shift — Question 15

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 :

  1. Option A:

    increased two times

    Correct
  2. Option B:

    increased 32\frac{3}{2} times

  3. Option C:

    decreased two times

  4. Option D:

    decreased 32\frac{3}{2} times

Answer: A

Step-by-step solution

Original focal length F0F_0 from biconvex: 1/F0=(n−1)(2/R)1/F_0 = (n-1)(2/R).

After cutting, plano-convex has 1/F0′=(n−1)(1/R)=1/(2F0)1/F_0' = (n-1)(1/R) = 1/(2F_0).

Magnification M∝L/F0M \propto L/F_0.

To keep M same, L/F0=L′/F0′L/F_0 = L'/F_0' ⇒ L′=L(F0′/F0)=L⋅2=2LL' = L (F_0'/F_0) = L \cdot 2 = 2L.

Answer key and solution verified before publishing.

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Exam
JEE Main 2026
Paper
5 April, Morning Shift
Subject
Physics
Chapter
Geometrical Optics
Topic
Optical Instruments

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