$(a)$ State two main causes of a person developing near-sightedness. How can this defect be corrected?
$(b)$ What is 'astigmatism' and how is this vision defect corrected?

(N/A) When a person can see near objects but is unable to see distant objects clearly,the defect is known as near-sightedness (myopia). This defect is caused by either $(i)$ excessive curvature of the cornea or $(ii)$ elongation of the eyeball. As a result,the image of a distant object is formed in front of the retina instead of on the retina. It can be corrected by using a concave lens of appropriate power.
$(b)$ Astigmatism is a vision defect where a person is unable to focus equally on objects in all directions (horizontal and vertical) simultaneously. This occurs because the cornea or the eye lens has different curvatures along different planes. This defect is corrected by using cylindrical lenses.