What is the power of a lens if the focal length is 0.75 m?
ANSWER: (A) 1.3 DREF: Khurana 4th Ed Pg 23The focal length (f) of a lens is the distance between the optical centre and the principal focus.Power of a lens (P) is defined as the ability of the lens to converge a beam of light falling on the lens. For a converging (convex) lens the power is taken as positive and for a diverging (concave) lens power is taken as negative.It is measured as reciprocal of the focal length in metres i.e. P = 1/f.The unit of power is dioptre (D). One dioptre is the power of a lens of focal length one metre.Hence, substituting in the formula, P=l/0.75 = 1,3 D
1.3 D
2.3 D
3.3
4.3 D
Elementary Optics