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. [2 MARKS]

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Solution

Problem: 2 Marks

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

$f = - 25 c m = - 0.25 m$

Power,

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

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