Question 39 If the power of a lens is − 4.0 D, then it means that the lens is a (A) concave lens of focal length −50 m (B) convex lens of focal length +50 cm (C) concave lens of focal length −25 cm (D) convex lens of focal length −25 m Now,
Power of lens = 𝟏/𝒇
Here,
focal length is in meter
Since
Power = −4D
𝟏/𝒇 = −4
(−1)/𝑓 = f
f = (−1)/4 m
Converting in cm
f = (−1)/4 × 100 cm
f = −25 cm
Since focal length is negative
∴ it is a concave lens
So, the correct answer is (C)

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CA Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Accounts and English at Teachoo