JEE Main · 2026 · 24 Jan 2026 · Shift 1 · Geometrical Optics

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In a microscope of tube length 10 cm two convex lenses are arranged with focal length of 2 cm and 5 cm. Total magnification obtained with this system for normal adjustment is

(5)^{k}

. The value of k is

(A)2✓ Correct
(B)5
(C)3.5
(D)4

Answer: (A)

Solution

In normal adjustment the magnifying power of a compound microscope is

M = \frac{L}{f _{o}} \cdot \frac{D}{f _{e}}

, where

L

is the tube length and

D = 25

cm is the least distance of distinct vision. With

L = 10

cm,

f_{o} = 2

cm and

f_{e} = 5

cm,

M = \frac{10}{2} \cdot \frac{25}{5} = 5 \times 5 = 25

(the same product is obtained if the two lenses are interchanged). Writing

25 = 5^{2}

and comparing with

5^{k}

gives

k = 2

JEE Main · 2026 · 24 Jan 2026 · Shift 1 · Single correct option

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