<prompt> What is the power of a lens if the focal length is 0.75 m?1.3 D2.3 D3.34.3 D  <response>: 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 1f.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, Pl0.75 1,3 D 