Question

what is astigmatism ? How is it caused and and how can it corrected?

Answer 1

Astigmatism is a problem of the eye caused by a defect in the spherical curvature of the eye leading to bluured vision. This condition leads to an eyes inability to focus light, since images are not formed on one point. This leads to blurred vision. It is caused when the eye's curvature (the cornea specifically) is not spherical but rather irregular.
Correction of astigmatism:
This condition is corrected by:
-Use of corrective eyeglasses
-Use of contact lenses
-Use of of corrective refractive surgery. 

Answer 2

In this defect, eye cannot focus object both in horizontal and vertical lines clearly.

Causes

This defect arise due to Imperfect shape of cornea on the lens of both.

Correction

This effect can be corrected by using cylindrical lens of refractive surgery.