A person is able to see objects clearly only when these are lying at distances between 50 cm and 300 cm from his eye.
(a) What kind of defect of vision is he suffering from?
(b) What kind of lenses will be required to increase his range of vision from 25 cm to infinity? Explain.
(a) For a normal eye, the near point is at 25 cm and the far point is at infinity. The given person cannot see object clearly either closer to the eye or-far away from the eye. So he is suffering from both myopia and hypermetropia.
(b) A bi-focal lens consisting of a concave lens and convex lens of suitable focal lengths will be required to correct the defects and to increase his range or vision from 25 cm to infinity. In a bi-focallens,theupperhalfofthelensisconcave which corrects distant vision and the lower half is convex which corrects near vision.