Question

Define power of a lens.write its si units.

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

Explanation:

We know that,

The power of the lens is the reciprocal of the focal length of the lens. The SI unit of the power is diopter. it is denoted by D. The power of the convex lens is positive and concave lens is negative.

1 D is the power of the lens whose focal length is 1 meter.

The power of the lens is

$P=\dfrac{1}{f}$

Here, P = power of the lens

f = focal length of the lens