Q.Give reasons for Cataract.

Refractive Errors of the Eye

To see clearly, the eye must bend (refract) incoming light so that it focuses exactly on the retina. When the eye's focusing power and its length are mismatched, light focuses in front of or behind the retina, and the image blurs. These focusing faults are the refractive errors, and each has a simple cause and a simple corrective lens.

The three common refractive errors

Myopia (short-sightedness)

• Problem: distant objects look blurred; near objects are clear.
• Cause: the eyeball is too long (or the lens too strong), so light from distant objects converges in front of the retina.
• Correction: a concave (diverging) lens, which spreads the light slightly before it enters, pushing the focus back onto the retina.

Hypermetropia (long-sightedness / hyperopia)

• Problem: near objects look blurred; distant objects are clearer.
• Cause: the eyeball is too short (or the lens too weak), so light from near objects would focus behind the retina.
• Correction: a convex (converging) lens, which bends light inward sooner, bringing the focus forward onto the retina.

Astigmatism

• Problem: distorted or blurred vision at all distances, in particular directions.
• Cause: the cornea (or lens) is unevenly curved — not perfectly spherical — so light in different planes focuses at different points.
• Correction: a cylindrical lens that compensates for the uneven curvature.

The summary table

| Defect | Cause | Image forms | Corrective lens | …