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JEE Main 2026 (2 Apr, Shift 2) · Spherical Surfaces & Thin Lenses

Spherical Surfaces & Thin Lenses question from JEE Main 2026 (2 Apr, Shift 2)

Q10JEE Main 2026 · 2 APR · SHIFT 2Medium

Spherical Surfaces & Thin Lenses

If sunlight is focused on a paper using convex lens, it starts burning the paper in shortest time when the lens is kept at 30 cm above the paper. If the radius of curvature of the lens is 60 cm then the refractive index of the lens material is α10\frac{\alpha}{10}. The value of α\alpha is ________.

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

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Answer: 20

  1. Paper burns fastest when it is at the focus: f=30f = 30 cm.
  2. Equiconvex lens, R=60R = 60 cm: 130=(μ−1)260\frac{1}{30} = (\mu - 1)\frac{2}{60}
  3. μ−1=1⇒μ=2.0=2010\mu - 1 = 1 \Rightarrow \mu = 2.0 = \frac{20}{10}
  4. So α=20\alpha = 20.
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