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JEE Main 2020 (8 Jan, Shift 1) · Spherical Surfaces & Thin Lenses

Spherical Surfaces & Thin Lenses question from JEE Main 2020 (8 Jan, Shift 1)

Q84JEE Main 2020 · 8 JAN · SHIFT 1Medium

Spherical Surfaces & Thin Lenses

A point object in air is in front of the curved surface of a plano-convex lens. The radius of curvature of the curved surface is 30 cm and the refractive index of the lens material is 1.5, then the focal length of the lens (in cm) is ______.

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

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Answer: 60

Lens maker's formula: $\frac{1}{f} = (\mu - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$ , plane side $R = \infty$ .
$\frac{1}{f} = 0.5 \times \frac{1}{30}$
So $f = 60$ cm.

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