21. What is meant by power of a lens? Write its SI unit. A student uses a lens of focal length 40 cm and another of –20 cm. Write the nature and power of each lens.
Power of lens is reciprocal of its focal length.
P = 1/f
SI unit of power = dioptre (D)
Here the convex lens has a positive focal length and the concave lens has a negative focal length.
Power of convex lens = P1 = 100 / 40 = 2.5 D
Power of concave lens = P2 = 100 /(-20) = - 5 D
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