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The thickness at the centre of a plano convex lens is 3 mm and the diameter is 6 cm. If the speed of light in the material of the lens is 2 × 10⁸ m s⁻¹, the focal length of the lens is

1. 0.30 cm
2. 15 cm
3. 1.5 cm
4. 30 cm

Correct answer: 30 cm

Solution

The focal length of a plano convex lens can be calculated using the lens maker's formula, which relates the curvature of the lens surfaces and the refractive index of the material. Given the thickness and diameter, the calculations yield a focal length of 30 cm, indicating that the lens has a relatively long focal distance.

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