A student uses a lens of focal length- 50 cm. What is the nature of the lens and its power?

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

P (in diopter) =

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

⇒ P =

Hence power of lens is -2 Dioptre.