JEE Main · 2026 · 24 Jan 2026 · Shift 2 · Geometrical Optics

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Distance between an object and three times magnified real image is 40 cm. The focal length of the mirror used is ____ cm.

(A) $- 15/2$
(B)-10
(C)-20
(D)-15✓ Correct

Answer: (D)

Solution

A real, inverted image three times the object size has magnification

m = - 3

, so

v = - 3 u

. Both the object and its real image lie in front of the mirror, and their separation is

∣ v ∣ - ∣ u ∣ = 3∣ u ∣ - ∣ u ∣ = 2∣ u ∣ = 40

cm, so

∣ u ∣ = 20

cm; thus

u = - 20

cm and

v = - 60

cm. The mirror formula gives

\frac{1}{f} = \frac{1}{v} + \frac{1}{u} = \frac{1}{- 60} + \frac{1}{- 20} = - \frac{1}{15}

. The focal length is

- 15

cm.

JEE Main · 2026 · 24 Jan 2026 · Shift 2 · Single correct option

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