Since the focal length is negative, therefore the lens is concave lens because the focal length of concave lens is negative.

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

F = -50cm = -0.5m (1m = 100cm)

⇒ P = -\(\frac{1}{0.5}\)= -2 D

**Hence power of lens is -2 Dioptre.**