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.

What is meant by power of a lens ? Write its SI unit. A student uses a

1.	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» SOLUTION :The power (P) of a lens is defined as the reciprocal of its focal length (f) expressed in metre. Thus, `P = (1)/(f ("in m"))` SI unit of power is dioptre (D), where 1 D = 1 `m^(-1)` The focal length of lens used by ONE student `f_(1) = + 40 CM = + 0.4`m `therefore` The lens is a convex lens and the power of lens `P_(1) = (1)/(f_(1)) = (1)/(0.4) = + 2.5` D Again the focal length of lens used by another student, `f_(2) = - 20 cm = - 0.2 m` `therefore` The lens is a concave lens and the power of lens `P_(2) = (1)/(f_(2)) = (1)/((-0.2)) = - 5.0` D