Power of lens is defined as **the efficiency with which a lens can converge or diverge the light ray. **

The reciprocal of focal length of the lens is called the power of the lens (P).

P (in dioptre) = \(\frac{1}{f(in\,meters)}\)

The S.I unit of power of lens is dioptre denoted by D.

f_{1} = 50cm = 0.5m (1m = 100cm)

⇒ P_{1} = 1/f_{1}

P_{1} = 1/0.5

P_{1} = 2D

Since Power is positive therefore the lens with focal length 50cm is convex lens.f_{2} = - 50cm = -0.5m (1m = 100cm)

⇒ P_{2} = 1/f_{1}

P_{2 }= 1/-0.5

P_{2} = -2D

Since Power is negative therefore the lens with focal length -50cm is concave lens. The image formed by the concave lens is virtual, erect and diminished irrespective of the position of the object.