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

- convex lens of focal length 2 m
- concave lens of focal length 50 cm
- concave lens of focal length 2 m
- convex lens of focal length 50 cm

**Answer**

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.**