Ans. is 'b' i.e., Non perpendicular principal meridians ASTIGMATISMo Astigmatism is a type of refractive error wherein the refraction varies in the different meridia. Consequently, the rays of light entering in the eye cannot converge to a point focus but form focal lines. The refractive error of the astigmatic eye stems from a difference in degree of curvature refraction of the two different meridians (i.e.,, the eye has different focal point in different planes). For example, the image may be clearly focused on retina in the horizontal plane, but not in the vertical plane.o The most common cause of astigmatism is abnormality of corneal curvature. Other less common causes are lenticular (curvature abnormality of lens,oblique position of lens) and retinal (oblique placement of macula). Types of astigmatismBased on axis of the principal meridiansRegular stigmatism : Principal meridians are perpendicularo With-the-rule astigmatism-the vertical meridian is steepest.o Against-the-rule astigmatism-the horizontal meridian is steepest.o Oblique astigmatism-the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.Irregular astigmatism - principal meridians are not perpendicular.Based on focus of the principal meridiansSimple astigmatismo Simple hyperopic astigmatism - first focal line is on retina while the second is located behind the retinao Simple myopic astigmatism - first focal line is in front of the retina while the second is on the retina.Compound astigmatismo Compound hyperopic astigmatism-both focal lines are located behind the retina,o Compound myopic astigmatism-both focal lines are located in front of the retina.Mixed astigmatism-focal lines are on both sides of the retina (straddling the retina).Treatment of Astigmatismo Treatment of astigmatism consists : -Optical treatment: - It consists of cylindrical power spectacles or contact lens. Types of contact lenses used are permeable contact tens, Soft toric contact lens (for high degree astigmatism), hybrid lens, i.e,, soft on hard lens, hard contact lens.Surgical treatment: - Non-laser (astigmatic keratotomy) or laser (PRK, LASIK).