THE HUMAN EYE

The human eye is one of the most valuable and sensitive sense organs, which enable us to see the colourful world around us. It is just like a photographic camera. The lens system of the eye forms an image of the object on a light sensitive screen.

STRUCTURE OF THE EYE

The eye is a hollow, spherical organ about 2.3 cm in diameter. The main parts of the human eye are Cornea, Iris, Pupil, Ciliary Muscles, Convex Lens, Retina and Optic Nerve.

The front part of the eye, which is transparent and bulging outwards is called as CORNEA.

Cornea serves as a window of the eye as the light coming from objects enters the eye through the cornea. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.

Behind the cornea, is a circular diaphragm called IRIS.

There is a hole in the middle of the iris which is called PUPIL of the eye.

Behind the pupil, is a convex lens called EYE LENS.

It is composed of transparent, fibrous jelly like material. It is held in position by CILIARY MUSCLES.

The focal length of the eye lens and hence converging power is not fixed.

It can be changed by changing its shape by action of ciliary muscles.

The eye lens forms an inverted real image of the object on the screen called RETINA.

The retina is behind the eye lens and at the back part of the eye.

The retina is a delicate membrane having enormous number of light sensitive cells – Rods and Cones.

The Rods respond to the intensity of light and the cones respond to colour of light.

These cells get activated upon illumination and generate electric signals to brain.

 

WORKING OF THE HUMAN EYE

Light rays coming from the object to be seen enter the eye through pupil and fall on the eye lens.

The eye lens being convex, forms a real and inverted image on the retina.

The light sensitive cell of the retina gets activated upon illumination and generate electric signals.

These signals are sent to the brain via the optic nerve.

The brain interprets these signals and finally give rise to the sensation of vision.

THE FUNCTION OF IRIS AND PUPIL

The iris adjusts the size of the pupil according to the intensity of light received by the eye.

The pupil regulates and controls the amount of light entering the eye.

When the intensity of outside light is high, the pupil contracts so that less light enters the eye.

When the intensity of the outside light is low, the pupil expands so that more light enters the eye.

POWER OF ACCOMODATION OF HUMAN EYE

A normal eye can see the nearby as well as distant objects clearly.

The eye lens is composed of a fibrous, jelly like material. Its curvature can be modified by the ciliary muscles.

The change in the curvature of the eye lens can thus change its focal length, which in turn changes the converging power of the eye lens.

When the eye is looking at the distant object, the ciliary muscles are relaxed, the eye lens becomes thin. Thus its focal length increases and converging power decreases.

Now the lens can converge the parallel rays of light coming from distant object to form the image on the retina. This enables us to see distant objects clearly.

When the same eye has to see nearby objects, the ciliary muscles contract.

This increases the curvature of eye lens. The eye lens becomes thicker. So, focal length of the eye lens decreases.

Due to short focal length, the converging power of the eye lens increases and now the eye lens can focus the diverging rays coming from the nearby objects on the retina.

The ability of an eye to focus the distant objects as well as nearby objects on the retina by adjusting its focal length is called accommodation.

However the focal length of the eye lens cannot be decreased below a certain minimum limit.

The maximum ‘accommodation‘ of a normal eye is reached when the object is at the distance of 25 cm of the eye.

Thus a normal human eye has a power of accommodation which enables objects as far as infinity and as close as 25 cm to be focused on the retina.

 

RANGE OF VISION OF A NORMAL HUMAN EYE

(i)    The minimum distance, at which objects can be seen most distinctly without strain is called least distance of distinct vision. It is also called near point of the eye. The near point of normal human eye is at a distance of 25 cm from the eye.

(ii)    The farthest point upto which the eye can see objects clearly is called the far point of the eye.

(iii)    The range of vision of a normal human eye is from infinity to about 25 cms.