Focal length of 0.25 m power of lens is ?
Ans. is 'c' i.e., 4 D Power of a lens (P) is defined as the ability of the lens to converge a beam of light falling on the lens. o For a converging (convex) lens the power is taken as positive and for a diverging (concave) ? ens 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. Therefore,Power of lens = 1/f = 1/0.25 = 4D Thus, A lens with a focal length of 0.25 m will be four times as strong as one with a focal length of 1 m: the refractive power of such a lens is therefore 4 dioptres (4 D).
40 D
1/4 D
4 D
25 D