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Lesson on the common eye defects

Answer Text:
Common eye defects
1. Short sightedness or hypermetropia
– the eyeball is too large for the ‘relaxed focal length’ of the eye. The defect is corrected by placing a concave lens in front of the eye.

2. Long sightedness or myopia
– images are formed beyond the retina. The defect is corrected by placing a converging lens in front of the eye.

3. Presbyopia
– this is the inability of the eye to accommodate and this occurs as the eye ages due to the weakening of the ciliary muscles. It can be corrected by the use a pair of spectacles.
4. Astigmatism
– this is a defect where the eye has two different focal lengths as a result of the cornea not being spherical. Corrected by the use of cylindrical lens.
5. Colour blindness– caused by deficiency of colour detecting cells in the retina.