A beam of light travelling parallel to the principal axis of a concave lens appears to diverge from a point 25 cm behind the lens after refraction. Calculate the power of the lens.

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Solution

When a parallel beam after refraction through the lens is incident on a concave lens, it appears to diverge from the focus of the lens. Hence, the focal length of the lens is 25 cm. According to sign convention, the focal length of a concave lens is negative.

f = - 25 cm = - 0.25 m

Power, P = $\frac{1}{f}$ = $\frac{1}{- 0.25}$ = -4 D

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