JEE Main · 2026 · 04 Apr 2026 · Shift 1 · Geometrical Optics

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A rod of length 10 cm lies along the principle axis of a concave mirror of focal length 10 cm as shown in figure. The length of the image is _____ cm.

(A)2.5
(B)5✓ Correct
(C)7.5
(D)7

Answer: (B)

Solution

Take the concave mirror's focal length as

f = - 10

cm. From the figure the near end of the rod is

20

cm from the mirror and the far end

30

cm, so the ends lie at

u = - 20

cm and

u = - 30

cm. Using

\frac{1}{v} + \frac{1}{u} = \frac{1}{f}

, the near end images at

v = \frac{u f}{u - f} = \frac{( - 20 ) ( - 10 )}{- 20 + 10} = - 20

cm and the far end at

v = \frac{( - 30 ) ( - 10 )}{- 30 + 10} = - 15

cm. The length of the image along the axis is

∣ - 20 - (- 15) ∣ = 5

cm.

JEE Main · 2026 · 04 Apr 2026 · Shift 1 · Single correct option

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