An optician prescribes a power = - 0.5 dioptre. The corresponding lens must be a

1. convex lens of focal length 2 m
2. concave lens of focal length 50 cm
3. concave lens of focal length 2 m
4. convex lens of focal length 50 cm

P = 1/f

f = 1/0.5 = 2 m

The power of a convex lens is positive and concave lens is negative.

The correct option is C.