Q 12-09-057JEE MainJEE Main 2026 (24 Jan, Shift 2)Easy
Distance between an object and three times magnified real image is 40 cm . The focal length of the mirror used is ______ cm .
Answer: (D) -15
A magnified real image is formed by a concave mirror, on the same side as the object: $m = -\dfrac vu = -3\Rightarrow v = 3u$.
Both distances are negative, so $|v| - |u| = 40\Rightarrow 2|u| = 40\Rightarrow u = -20$ cm, $v = -60$ cm.
$$\frac1f = \frac1v + \frac1u = -\frac1{60} - \frac1{20} = -\frac4{60}\Rightarrow f = -15\ \text{cm}$$
Solution by Sreeraj P, M.Sc Physics