What is the defect of vision in the human eye? State its types and explain in detail.

Defects of human eye is simply the inability of the ciliary muscles to change the thickness of the eye lens. They can be classified into three types:

1. Near-sightedness: Myopia or near sightedness is an eye defect in which a person can see clearly only when the objects are near. This happens when the lens becomes too thick and the light rays undergo greater refraction which makes the light rays to converge before the retina, making the eyesight blurry.

Using a concave lens can be helpful as it concentrates the light rays on the retina. The below image explains the process

Fig. Correction of myopia

2. Hypermetropia: Hypermetropia or far-sightedness is an eye defect there is difficulty with near vision but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hypermetropia can be corrected by wearing glasses/contacts that contain convex lenses.

Fig. Correction of Hypermetropia

3. Presbyopia: Presbyopia is the decrease in accommodation power with ageing which can be corrected by bifocal lenses. This defect is caused due to weakening of ciliary muscles and loss of elasticity of the eye lens. This can be treated by using bifocal lens.