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Answers
a )1. myopia or near sightedness is a eye defect in which a person can see nearby objects clearly but cannot see distant objects
4 .hypermetropia or far sighttedness is eye defect in which a person can see distant objects clearly but cannot see nearby objects
(i) MYOPIA
=it is also known as short sightedness . In this disease the person is not able to see far off object clearly.
They can be corrected by using concave lenses.
(ii) ASTIGMATISM
=Astigmatism usually is caused by an irregularly shaped cornea. In an eye with astigmatism, light fails to come to a single focus on the retina to produce clear vision. Instead, multiple focus points occur, either in front of the retina or behind it (or both).
They can be corrected by using cylinderical lenses.
(iii)BIFOCAL LENSES
=These are the lenses that are used when the person is suffering from presbiopia.In this, the upper part consistes of concave lens and lower part contains convex lense.
(iv)FAR SIGHTEDNESS
=This is also known as hypermetropia . In this the person is not able to see near by objects clearly (under25cm).They can be corrected by using convex lens.