CBSE Class 10 Answered

Like nearsightedness and farsightedness, astigmatism is a refractive error, meaning it is not an eye disease or eye health problem; it's simply a problem with how the eye focuses light.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).

Astigmatism usually causes vision to be blurred or distorted to some degree at all distances.

Astigmatism usually is caused by an irregularly shaped cornea. Instead of the cornea having a symmetrically round shape, it is shaped more like an ellipsoid. As we know, unlike symmetrical diameter in sphere, ellipsoidal shape has two major diameters, namely major diameter and minor diameter. These diameters are perpendicular. Let us call these diamters as meridian. In an eye having the defect of astigmatism,  one meridian being significantly more curved than the meridian perpendicular to it.

In addition to the spherical lens power used to correct nearsightedness or farsightedness, astigmatism requires an additional "cylinder" lens power to correct the difference between the powers of the two principal meridians of the eye.