What is meant by power of a lens ? Write its Sl unit. A student uses a lens of focal length 40 cm and another of - 20 cm. Write the nature and power of each lens.

Explanation

The degree of convergence or divergence of a lens is called the power of a lens (P). OR Power of a lens is defined as the reciprocal of focal length of a lens.
Power of lens (P) `=(l)/("Focal length of the lens in metre (f)")`
`P=(l)/(f)`
The SI unit of power of lens is expressed in dioptre (D).
`{:("Lens 1","Lens 2"),(f=+40 cm=+0.40 m,f=-20cm=-0.2m),(P=(l)/(f)=(1)/(0.4)=+2.5 D,P=(l)/(f)=(1)/(0.2)=+5D),("It is a convex lens, converges incident light.","It is a concave lens, diverges incident light."):}`