A person cannot see distinctly objects kept beyond 2 m This defect can be corrected by using a lens ......

A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power 

If a person cannot see distinctly, objects kept beyond 21 m, then he is suffering from myopia.

As the person has the eye defect, myopia, therefore a concave lens has to be used whose focal length will be f = -2 m (using sign convention). Thus,
Power, P = 1/f [where, f is focal length in metre.]
= 1/-2 = -0.5D 

By: santosh