Diameter of a plano-convex lens is 6 cm and thickness at the center is 3 mm. If speed of light in material of lens is 2 × 10⁸ m/s, the focal length of the lens is (a) 15 cm (b) 20 cm (c) 30 cm (d) 10 cm

Answer: (c). μ = c/v = (3 × 10⁸)/(2 × 10⁸) = 1.5 Sag: R ≈ r²/2t = (3²)/(2 × 0.3) = 15 cm. f = (R)/(μ - 1) = 15/0.5 = 30 cm So the answer is (c) 30 cm.

Diameter of a plano-convex lens is 6 cm and thickness at the center… | JEE Main 2013

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Spherical Surfaces & Thin Lenses

Diameter of a plano-convex lens is $6\text{ cm}$ and thickness at the center is $3\text{ mm}$ . If speed of light in material of lens is $2 \times 10^{8}\text{ m/s},$ the focal length of the lens is

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Answer: (c)

$\mu = \frac{c}{v} = \frac{3 \times 10^8}{2 \times 10^8} = 1.5$
Sag: $R \approx \frac{r^2}{2t} = \frac{3^2}{2 \times 0.3} = 15$ cm.
$f = \frac{R}{\mu - 1} = \frac{15}{0.5} = 30$ cm
So the answer is (c) 30 cm.

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