First slide

The lens

Question

The radius of curvature for a convex lens is 40 cm, for each surface. Its refractive index is 1.5. The focal length will be

Moderate

A

40 cm
B

20 cm
C

80 cm
D

30 cm

Solution

$By formula \frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}}) = (1 .5 - 1) (\frac{1}{40} + \frac{1}{40}) = 0 .5 \times \frac{1}{20} = \frac{1}{40} ∴ f = 40 cm$

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