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JEE Main 2021 (25 Feb, Shift 1) · Spherical Surfaces & Thin Lenses

Spherical Surfaces & Thin Lenses question from JEE Main 2021 (25 Feb, Shift 1)

Q73JEE Main 2021 · 25 FEB · SHIFT 1Easy

Spherical Surfaces & Thin Lenses

The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is __________ cm.

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

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Answer: 15

  1. For a lens m=ff+um = \frac{f}{f + u}. Same size means one image is real and the other virtual: m1=−m2m_1 = -m_2.
  2. ff−20=−ff−10\frac{f}{f - 20} = -\frac{f}{f - 10}
  3. f−10=20−f⇒2f=30f - 10 = 20 - f \Rightarrow 2f = 30
  4. f=15 cmf = 15\text{ cm}
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