The Human eye | its category | its defects

HUMAN EYE

the human eye is like a camera having a transparent convex lens on one side and a sensitive screen called the retina on the other. The essential parts are shown in the figure. The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.

• The front transparent part of the eye is called cornea. It is bulged outwards. Cornea acts as the window of the eye as light from the object to be seen enters the eye through cornea and most of the refraction for these light occurs at the outer surface of the cornea.

• Behind the cornea there is a dark muscular diaphragm. It is called iris. There is a hole in the iris called pupil. The iris controls the size of the pupil. It adjust the size of the pupil according to the intensity of the light. When intensity of outside light is high, the pupil contract so that less light enters the eye, when intensity of light is low, the pupil expands so that more light enters the eye.

• Behind the pupil there is a double convex lens. This lens is made up of a fibrous, jelly like material and is held in position by the ciliary muscles.

• The eye lens is different from a lens made of glass in the sense that a glass lens has a fixed thickness and therefore a fixed focal length while the thickness of the eye lens and hence its focal length is adjustable. It can be modified by the ciliary muscles.

• The space between the cornea and the eye lens is filled with a viscous liquid, called aqueous humour, and the space between eye lens and retina contains a transparent jelly called vitreous humour.

•Working. The light enters through the transparent cornea, passes through the lens and focussed on the ratina. The retina is sensitive to light and sends messages to the brain by way of the optic nerve. An inverted images is formed on the retina but the brain perceived it is as the object is, that is without inversion.

POWER OF ACCOMMODATION

Power of accommodation of the eye is the ability of the eye to observe distinctly the objects situated at widely different distances from the eye. This is made possible by the adjustment of the focal length of the eye lens by the action of ciliary muscles holding the lens.

• The point at shortest distance of any object from the eye to be able to see the object clearly.

Least distance of distant vision:- it is the distance of near point from the NORMAL EYE and is equal to 25cm.

Far point:- It is the distance of any object from the eye to be able to see the object clearly. For a NORMAL EYE the far point is at infinity.

PERSISTENCE of VISION

The image of an object viewed persists on the ratina for 1/16 second, even after the removal of the object. This is called persistence of vision.

The property of persistence of vision is used in cinematography. The sequence of still pictures taken by a movie camera is projected on a screen at a rate of about 24 images or more per second. The successive impressions of images on the screen appears to merge smoothly into another to give the viewer a feeling of moving images.

DEFECT of VISION

• Ability to see the object is called vision. We also call it eye sight.

• The figure shows normal vision.

• The human eye is able to focus the image for different objects at different distances by changing the focal length of the lens.

• When a person is not able to see clearly the distant or the nearly objects, then is said to have defect of vision.

There are three defect of vision is -

1- Hypermetropia or long-sightedness or far-sightedness

2- Short-sightedness or myopia

3- Presbyopia or Old sight

1:- Hypermetropia:-

• The near point that is the point at the closest distance, at which an object can be seen clearly by the eye is 25 cm.

• A person suffering from hypermetropia is able to see clearly the distant object but not the near object as the image of the object falls behind the retina. Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading. This defect arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small.

• This defect can be corrected by placing a convex lens of appropriate focal length in front of the eye this converging lens provides the additional focussing power required for forming the image on the retina.

2 :- Myopia

• A short sighted person can see clearly but cannot see the distant objects distinctly. The image of a distant object is formed in front of the retina but not at the retina itself. This defect may arise due to the eye-ball becoming or due to excessive curvature of the eye lens.

• This can be rectified by using spectacles with concave lens of appropriate focal length. The concave lens will bring the image back of the retina.

3:- Presbyopia

• it is that defect of human eye, due to which an old is not able to read or write comfortably. That is why, presbyopia is also called old sight.

• Cause of presbyopia:- As the age advances, the ciliary muscles weaken and eye lens loses its ability to abjust it's focal length. As a result, the near point recedes to a distance more tha 25 cm from the eye and so an old person suffering from presbyopia finds it difficult to see nearly objects distinctly without using corrective eye glasses. This, presbyopia is a sort of hypermetropia. Hence and old person suffering from presbyopia needs to ise spectacles with convex lens of suitable focal length.

• It may be noted that hypermetropia is caused due to either decrease in length of eye ball or increase in focal length of eye lens but the cause of presbyopia is only increase in focal length of eye lens. The eye ball, in presbyopia has normal length.

- Colour blindness

A person suffer from colour blindness when the person is not able to distinguish between colours, through his or her vision may otherwise be normal.

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