In senile nuclear cataract what type of myopia is seen-

Ans. is 'b' i.e., Index myopia o Nuclear changes of aging induce a modification of refractive index of lens and produce an index myopia."Nuclear cataracts cause a general decrease in the transperancy of the lens nucleus. They are associated with index myopia "-- Ophthalmic study guideCauses of errors of refractiono Possible causes of ametropia are : -AxialIt is the commonest form of ametropia (both myopia and hypermetropia). In hypermetropia, there is an axial shortening of eyeball. So, image is formed behind the retina. In myopia, there is an axial lengthening of eyeball. So, image is formed in front of the retina. 1 mm change in axial length leads to ametropia of 3D. For example 1 mm shortening in axial length causes hypermetropia of 3D.CurvatureChange in the curvature of cornea or lens will cause ametropia. In hypermetropiaf the curvature of cornea or lens is lesser than normal. In myopia, there is increase in curvature of cornea or lens./ mm change in corneal curvature leads to 6-7 D ametropia.IndexIf refractive index of optical system is low, it will result in hypermetropia and high refractive index will result in myopia.Positional (Due to relative position of the lens)A forward shift of lens causes myopia, backward shift result in hypermetropia. Absence of lens (aphakia) causes hypermetropia.Excessive accommodationExcessive accommodation due to spasm of accommodation causes myopia.