Question

9. 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.​

Answer 1

power of lens = reciprocal of focal length. i.e 1/f

SI UNIT =dioptre

Answer 2

The reciprocal of focal length is defined as the power of a lens. Its SI unit is Dioptre which is shown by capital D .

Case 1st,

focal length(f)= 40 cm , which is of convex lens

since,

p=1/f,

thus,

p=1/40

p=0.025 D

hence, mirror used in this case is converging in nature.

Case 2nd

focal length(f)= -20 cm , which is of concave lens

since,

p=1/f

thus

p=1/-20

p= -0.05 D

hence, mirror used in this is diverging in nature.