What is meant by power of a lens? Name and define its S.I. unit.

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

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

f1 = 50cm = 0.5m

P1 = 1/f1

P1 = 1/0.5

P1 = 2D

Since Power is +ve therefore the lens with focal lenght 50cm is convex lens.

f2 = -50cm = -0.5m

P2 = 1/f1

P2 = 1/-0.5

P2 = -2D

Since Power is -ve therefore the lens with focal lenght -50cm is concave lens.

The image formed by the concave lens is virtual, erect and diminished irrespective of the position of the object.