Ray Optics

JEE Main level, single correct, medium

20 of 295

Question 20 of 295

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

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Answer: (B) -15

For a mirror with magnification m = -3 (real image), object and image distances satisfy |u| + |v| = 40 cm.
Since m = v/u = -3, we have v = -3u, giving u + 3u = 40, so u = 10 cm and v = -30 cm.
Using mirror formula: 1f=1u+1v=110+1−30=3−130=115\frac{1}{f} = \frac{1}{u} + \frac{1}{v} = \frac{1}{10} + \frac{1}{-30} = \frac{3-1}{30} = \frac{1}{15}
Hence f = -15 cm.

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