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A plano-convex lens is made of material of refractive index 1.6. The radius of curvature of the curved surface is 60 cm. The focal length of the lens is

1. 50 cm
2. 100 cm
3. 200 cm
4. 400 cm

Correct answer: 200 cm

Solution

The focal length of a plano-convex lens is given by the lens maker's formula: \( \frac{1}{f} = (\mu - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \). Here, \( \mu = 1.6 \), \( R_1 = 60 \ \text{cm} \), and \( R_2 = \infty \) (since the plano side is flat). Substituting, \( \frac{1}{f} = (1.6 - 1) \left( \frac{1}{60} - 0 \right) = \frac{0.6}{60} = \frac{1}{100} \). Thus, \( f = 100 \ \text{cm} \).

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