Question

define power of lens & state it SI unit​

Answer 1

The ability of a lens to converge/diverge a beam of light rays is expressed in terms of its Power(P).

P=1/f

The SI unit of power of lens is Dioptre.It is denoted by the letter "D"

SI unit of length is metre(m)

therefore

1D = 1/m or 1D = 1m-1 

Answer 2

Hi!!

Power of lens - The power of a lens is determined by the reciprocal of its                      

                          focal length.

Power of lens = 1 / Focal length ( in meters )

Its SI unit is Diopter (D)

Concave lens has a power of negative (-) while the convex lens has a power of positive (+).