Question

Question 1
A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power

(a) + 0.5 D
(b) – 0.5 D
(c) + 0.2 D
(d) – 0.2 D

Answer 1

Option (b) is correct.
Since the person is not able to see distant objects clearly, he is suffering from Myopia. Myopia can be corrcted using concave lenses which have negative power and negative focal lengths.
Since the person is not able to see beyond 2 m, he'll need a lens which to produce image, of objects kept at 2m, at infinity.

Note: To form image at infinity, object needs to be at focus (u = f).
$P=\frac{1}{f}$
$\Rightarrow P=\frac{1}{2m}=0.5D$
A concave lens must be used, therefore power should be –0.5 D