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JEE Main 2013 (22 Apr, Shift 1) · Spherical Surfaces & Thin Lenses

Spherical Surfaces & Thin Lenses question from JEE Main 2013 (22 Apr, Shift 1)

Q99JEE Main 2013 · 22 APR · SHIFT 1Medium

Spherical Surfaces & Thin Lenses

The image of an illuminated square is obtained on a screen with the help of a converging lens. The distance of the square from the lens is 40 cm. The area of the image is 9 times that of the square. The focal length of the lens is:

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Answer: (d)

  1. Area ratio 9 means linear magnification ∣m∣=3|m| = 3; real image, so v=3×40=120v = 3 \times 40 = 120 cm.
  2. 1f=1120+140=4120\frac{1}{f} = \frac{1}{120} + \frac{1}{40} = \frac{4}{120}
  3. f=30f = 30 cm
  4. So the answer is (d) 30 cm.
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