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JEE Main 2015 (11 Apr, Shift 1) · Combination of Lenses

Combination of Lenses question from JEE Main 2015 (11 Apr, Shift 1)

Q72JEE Main 2015 · 11 APR · SHIFT 1Medium

Combination of Lenses

A thin convex lens of focal length 'f' is put on a plane mirror as shown in the figure. When an object is kept at a distance 'a' from the lens −- mirror combination, its image is formed at a distance a3\frac{a}{3} in front of the combination. The value of 'a' is:

Figure

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Answer: (b)

  1. Lens on a plane mirror acts as a concave mirror of focal length F=f2F = \frac{f}{2}.
  2. Real image in front: 1v+1u=1F\frac{1}{v} + \frac{1}{u} = \frac{1}{F} with u=−au = -a, v=−a3v = -\frac{a}{3}, F=−f2F = -\frac{f}{2}.
  3. 3a+1a=2f⇒a=2f\frac{3}{a} + \frac{1}{a} = \frac{2}{f} \Rightarrow a = 2f
  4. So the answer is (b) 2f2f.
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