(a) Explain the following terms used in relation to defects of vision and corrections provided for them: (i)…
(a) Explain the following terms used in relation to defects of vision and corrections provided for them:
(i) Myopia
(ii) Astigmatism
(iii) Bifocal lenses
(iv) Far sightedness.
(b) Describe with a ray diagram how a person with myopia can be helped by spectacles.
Solution
(a) (i) Myopia is a kind of defect in human eye due to which a person can see near by objects clearly but unable to see the distant objects. Myopia is due to (i) excessive curvature of cornea (ii) elongation of eye ball.
(ii) Astigmatism is a kind of human eye defect in which he cannot see simultaneously horizontal and vertical lines both. This defect arises either when the cornea or the eye lens or both are not perfectly spherical. It can be corrected by using cylindrical lenses.
(iii) Bifocal lenses with the ageing a person may suffer myopia and hypermetropia. For correction of these defects he is advised to use bifocal lenses whose upper half is concave and lower part is convex to see distant and reading purposes respectively.
(iv) Far sightedness, a person suffering from this defect can see distant objects but unable to see nearby objects clearly. The image of an objects is formed beyond the retina. Such defects can be corrected by using a convex lens.
(b) A person with myopic eye may be advice to use a concave lens of suitable power/ focal length.
