Find out the focal length of a lens of power D

The power of the lens is known as the inverse of the focal length of the lens measured in meter(m).

Power of lense = 1/ focal length in meter.

Power of lens (P) = 1/f

P = -2D

f = -1/2 = -0.5 m

A concave lens has a negative focal length. Therefore, it is a concave lens.

Power of lens

The power of a lens is the measure of the degree of convergence or divergence of the light rays falling on it. The degree of convergence or divergence depends upon the focal length of the lens. Thus, we define the power of the lens as the reciprocal of the focal length of the lens used. It is given as,

Power of lens (P) = 1/f

Where f is the focal length of the lens used. SI unit of power is Dioptre (D). The power of the concave lens is negative, while the power of the convex lens can be positive.