The focal length of the lens is 0*75 m, what will be its refractive power -: (A) 0.75D, (B) 1.5D, (C) 1.33 D, (D) 1.25 D
Answer is C. The 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.
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.