The distance between object and its 3 times magnified virtual image as produced by a convex lens is 20 cm. The focal length of the lens used is __________ cm. Numerical answer type. Work it out, then open the answer.

Answer: 15. Virtual image on the same side: v = 3u and |v| - |u| = 20 cm. 2|u| = 20 ⇒ u = -10 cm, v = -30 cm 1/f = 1/v - 1/u = -1/30 + 1/10 = 2/30 f = 15 cm

The distance between object and its 3 times magnified virtual image… | JEE Main 2024 (1 Feb, Shift 1)

Q46JEE Main 2024 · 1 FEB · SHIFT 1Medium

Spherical Surfaces & Thin Lenses

The distance between object and its 3 times magnified virtual image as produced by a convex lens is $20\text{ cm}$ . The focal length of the lens used is __________ $\text{cm}$ .

Numerical answer type. Work it out, then open the answer.

Show answer and solution

Answer: 15

Virtual image on the same side: $v = 3u$ and $|v| - |u| = 20$ cm.
$2|u| = 20 \Rightarrow u = -10$ cm, $v = -30$ cm
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u} = -\frac{1}{30} + \frac{1}{10} = \frac{2}{30}$
$f = 15$ cm

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