The Human Eye

Structure, Defects and Correction

Eye is one of the most important sense organs. It is the eye which enables us to see and understand the colourful world around us. There are many persons who cannot see and are called blind.

Eye works like a camera, rather camera was invented after observing the working pattern of eye and it can be said that camera works like an eye.

The Human Eye

Structure of the human eye

The structure of human eye is similar to a spherical ball. Light coming from object enters into eye ball through the lens and cast the inverted image at retina, which is a light sensitive screen located at the back portion of eye ball. Optic nerves send this image to the brain which interprets the correct picture and we see the object or anything.

Eye Ball

The whole eye is generally called eye ball. Human Eye ball is approximately spherical in shape. It has a diameter of about 2.3 cm. There are several parts of eye ball.

Cornea

Cornea is a thin membrane and forms the transparent bulge on the front surface of the eye ball. Cornea covers the iris, pupil and anterior chamber. Transparency is the prime importance of cornea. Cornea does not have blood vessels. Cornea receives nutrients via diffusion from the tear fluid from outside surface and the aqueous humour through the inside surface.

Most of the refraction for light rays coming from an object entering the eye occurs at the outer surface of cornea.

Iris

Iris is a thin, circular and coloured structure located just behind cornea. The plural of word 'Iris' is 'Irides or Irises'. Iris is dark muscular diaphragm that controls the diameter and size of pupil and thus the amount of light reaching the retina. Iris contains pigment which defines eye colour of a person.

Pupil

Pupil is the hole located in the centre of iris of the eye. Pupil regulates and controls the amount of light and allows light to strike the retina. The shape of human pupil is round.

Lens or Crystalline Lens

Eye lens is similar to other lenses. Eye has a transparent biconvex lens composed of fibrous jelly like materials and is located just behind the iris. Eye lens forms a real inverted image of the object on the retina.

Retina

Retina is the inner back portion of eye. Retina is composed of enormous number of light sensitive cells. Lens forms image of the visual world on retina through cornea. Light sensitive cells of retina get activated upon illumination and generate electric signals. Optic nerves send these electric signals to the brain, where these signals finally processed and interpreted to see an object as they are.

Power of Accommodation

The curvature of eye lens can be modified according to the distance of object by ciliary muscles with which lens is attached. By changing the curvature focal length of eye lens is changed.

Contraction of ciliary muscles make eye lens thicker which decreases the curvature of eye lens consequently focal length decreases. This enables us to see nearby objects clearly.

Relaxation of ciliary muscles make eye lens thin which increases the curvature of eye lens which increases its focal length. Increase in focal length of eye lens enables us to see object at distance clearly.

The ability of eye lens to adjust its focal length is called accommodation or power of accommodation.

But focal length of eye lens can be decreased to only a certain extent. Eye lens has a minimum limit to decrease its focal length. This limit is 25cm from eyes. It you try to see an object below the distance of 25cm from eye, object appears blurred.

Least Distance of Distinct Vision or Near Point of Eye

The minimum distance, at which objects can be seen most distinctly without any strain, is called the least distance of distinct vision. Least distance of distinct vision is known as near point of the eye also.

Far Point of Eye

The farthest point upto which the eye can see objects clearly is called the far point of the eye. Far point of a normal eye is between 25cm to infinity.

Defects of Vision and their Correction

Sometimes the eye may gradually lose its power of accommodation and the person cannot see the objects clearly and comfortably. This happens because of refractive defects of the eye because of losing of power of accommodation of lens.

There are mainly three types of defects of vision these are Myopia, Hypermetropia and Presbyopia.

Myopia or Near Sightedness or Short Sightedness

Myopia is known as Near Sightedness or Short Sightedness also. A person suffering from myopia can see objects placed nearby clearly but cannot see the objects placed at a distance clearly.

Cause of Mypoia Near Sightedness or Short Sightedness

The far point of a person suffering from Myopia or Near Sightedness becomes nearer than infinity.

This problem may arise because of

(a) excessive curvature of the eye lens

(b) elongation of eye ball.

Because of excessive curvature of the eye lens or elongation of eye ball image of object placed at infinity formed before retina instead of retina and object appears blurred or nor visible.

Correction of Mypoia Near Sightedness or Short Sightedness

Mypoia or Near Sightedness or Short Sightedness can be corrected by using a diverging lens (Concave lens) of suitable power. Converging lens diverge rays of light coming from the object and object appears placed farther than actual place. This makes image at retina which enables a person having Myopic eye defect see clearly.

This eye defect Myopia known as Near Sightedness or Short Sightedness also because person suffering from this defect has far point becomes nearer.

Hypermetropia or Far Sightedness

Hypermetropia is known as Far Sightedness also. A person's eye suffering from Hypermetropia can see an object placed at a distance but cannot see that placed at nearby clearly. He sees object placed at nearly blurred.

Cause of Hypermetropia or Far Sightedness

In this defect of Hypermetropia or Far Sightedness the near point of a person suffering from this becomes father than 25cm which is least distance of distinct vision. This defect of Hypermetropia or Far sightedness may arise due to

(a) the decrease in curvature of eye lens which makes focal length of eye lens too long

(b) the eyeball becomes smaller than normal.

Because of long focal length or small eye ball image of an object placed at normal distance formed behind the retina and person suffering from this defect sees that blurred image of object.

Correction of Hypermetropia or Far Sightedness

This defect can be corrected using a converging lens (Convex lens) of appropriate power. Converging lens converse the light rays coming from object and object appear placed at more distance than normal. Because converging of light rays coming from object image formed at retina and person suffering from this defect sees the object clearly.

This defect of Hypermetropia is known as Far sightedness because focal length of eye lens becomes far than normal i.e. increases.

Presbyopia

Person suffering from presbyopia find it difficult to see the objects placed nearby comfortably and distinctly.

Cause of Presbyopia

The defect of Presbyopia arises because of ageing in people. With increase in age the flexibility of ciliary muscle and eye lens gets weakens because of these ciliary muscles and eye lens become unable to adjust to the suitable focal length and cannot form the image of object at retina. Because of this defect image of object placed nearby is not formed at retina and person suffering from Presbyopia cannot see the object placed nearby clearly.

Correction of the eye defect of Presbyopia

The defect of Presbyopia is corrected by using a converging lens (Convex lens). Convex lens converge light rays coming from object placed nearby and object appears slightly far from the actual place and image formed at retina. After correction person suffering from Presbyopia can see the object clearly.

Bifocal Lens

Bifocal lens means lens with dual focus. Such bifocal lenses are made of convex and concave both lenses. Concave lens is fitted at the upper part while convex lens is fitted for lower part of bifocal lens.

Upper part of lens is meant to see the object placed at distance while lower part of lens is used while reading or see the object placed nearby.

Sometimes a person may suffer from both of the myopoia and hypermetropia defects. In such case bifocal lens is required to correct the vision.

These all types of defects of eye are called refractive defects as in such case rays of light coming from object cannot refracted from eye lens properly.

These days the refractive defects of eye can be corrected using contact lens or through surgery. Contact lens is similar to lenses fit in the spectacles. But contact lens is directly placed at the surface of eye while normal lenses are fitted in spectacles.