A man can read the number of a distant bus clearly but he finds difficulty in reading a book.
(a) From which defect of the eye is he suffering ?
(b) What type of spectacle lens should he use to correct the defect ?

Explanation

The person is suffering from hypermetropia or long-sightedness.
A long-sighted person can see distant objects clearly, but cannot see distinctly objects lying closer than a certain distance. Thus, he cannot see clearly an object lying at 25 cm which is the least distance of distinct vision for a normal eye. His far point is farther from the eye than 25 cm, say at 75 cm or so.
Causes of the Defect. This defect is due to either :
(i) the size of the eye ball becoming too short.
(ii) the lens becoming too him , so that its focal length becomes abnormally large .
Normal eye is able to focus on retina the rays emerging our from N.
However, the defective eye is not able to focus the rays from near point of normal eye.
It can focus the rays from near point of defective eye. From we conclude that more inclined rays are not focussed on reetina.
Correction of this Defect. Such a defect is corrected by placing a convex or converging lens of suitable focal length before the eye so that the rays diverging from N appear to come after refraction, from the near point .