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Structure, Function and Range of Vision of The Human Eye
Eye is one of our most important sense organs. The eye enables us to see the various objects around us. The main parts of the human eye are : Cornea, Iris, Pupil, Ciliary muscles, Eye lens (which is a flexible convex lens), Retina and Optic nerve. All these parts of the eye are shown in the simplified diagram of human eye given in Figure. We will now describe the construction and working of the human eye.
Construction of the Eye
Our eye is shaped like a ball. It has a roughly spherical shape. The outer coat of the eye is white. The outer coat of the eye/is tough so that it can protect the interior of the eye from damage during an accident. The front part of eye is called cornea (see Figure). Cornea is made of a transparent substance and it is bulging out. The light coming from an object enters the eye through cornea.
The main function of cornea is to protect the eye but it also helps in focussing some light (by its converging action). Just behind the cornea is the ‘Iris’. Iris is the coloured part of the eye. The iris has a hole at its centre which is called pupil. Pupil appears like a dark spot in the centre of iris because no light is reflected from it. Iris controls the amount of light entering the eye by adjusting the size of pupil. The iris is actually that part of the eye which gives the eye its distinctive colour. When we say that a person has green eyes, we mean that the colour of iris in his eyes is green.
Behind pupil is the eye-lens (see Figure). The eye- lens is a convex lens made of a transparent and flexible material like jelly. The eye-lens is actually a living lens because it is made up of transparent living cells which allow light to pass through them. The eye-lens is held in position by ciliary muscles whose one side is attached to the eye-lens and the other side is attached to the eye-ball. The ciliary muscles can change the curvature of eye-lens and make it thin or thick according to the need of the eye.
In other words, the ciliary muscles can change the focal length (and hence converging power) of the eye-lens according to the requirements of the eye to see distant objects or nearby objects. Thus, the focal length (and hence converging power) of the eye-lens is controlled by ciliary muscles. Please note the difference between convex eye-lens and the ordinary convex lens made of glass: The eye-lens is a flexible convex lens whose thickness and hence focal length (or converging power) can be changed by the action of ciliary muscles. On the other hand, a glass convex lens has a fixed thickness due to which its focal length (or converging power) is also fixed, and cannot be changed.
The retina is a screen on which the image is formed in the eye. The retina is behind the eye-lens, at the back part of the eye (see Figure). The eye-lens focusses the image of an object on the retina. The retina is attached to optic nerve (see Figure). The optic nerve carries the image formed on retina to the brain in the form of electrical signals. The space between cornea and eye-lens is filled with a viscous liquid called ‘aqueous humour’.
And the space between eye-lens and retina is filled with another liquid called ‘vitreous humour’. We will discuss aqueous humour and vitreous humour in higher classes. The eyes also have eyelids which prevent any object from entering the eyes. The eyelids also shut out light when not required. We will now describe the working of human eye.
Working of the Eye
When we look at an object, then light rays coming from the object enter the pupil of the eye and fall on the eye-lens. The eye-lens is a convex lens, so it converges the light rays and produces a real and inverted image of the object on the retina. The retina has a large number of light sensitive cells. When the image of the object falls on the retina, then the light sensitive cells generate electrical signals. The retina sends these electrical signals to the brain through the optic nerve and we are able to see the object. Please note that although the image of an object formed on the retina is inverted, but our brain interprets this image as that of an erect object.
The Function of Iris and Pupil
The iris automatically adjusts the size of pupil according to the intensity of light received by the eye from the surroundings. If the amount of light around us is very high (as during the day light), then iris contracts the pupil (makes the pupil small) and hence reduces the amount of light entering the eye (see Figure). On the other hand, if the amount of light around us is small (as in a dark room or during the night) then iris expands the pupil (makes the pupil large) so that more light can enter the eye (see Figure). This is because we need to allow more light to go into the eyes when the outside light is dim so as to see properly.
It should be noted that the adjustment of the size of pupil takes some time. For example, when we go from a bright light to a darkened cinema hall, at first we cannot see our surroundings clearly. After a short time our vision improves, and we can see the persons sitting around us.
This is due to the fact that in bright sunlight the pupil of our eye is small. So, when we enter the cinema hall, very little light enters our eye and we cannot see properly. After a short time, the pupil of our eye expands and becomes large. More light then enters our eye and we can see clearly.
On the other hand, if we go from a dark room into bright sunlight or switch on a bright lamp, then we feel the glare in our eyes. This is due to the fact that in a dark room, the pupil of our eye is large. So, when we go out in bright sunlight or switch on a bright lamp, a large amount of light enters our eyes and we feel the glare. Gradually, the pupil of our eye contracts. Less light then enters our eye and we can see clearly.
Rods and Cones
The retina of our eye has a large number of light-sensitive cells. There are two kinds of light-sensitive cells on the retina : rods and cones.
(i) Rods are the rod-shaped cells present in the retina of an eye which are sensitive to dim light. Rods are the most important for vision in dim light (as during the night). Rod cells of the retina, however, do not provide information about the colour of the object.
(ii) Cones are the cone-shaped cells present in the retina of an eye which are sensitive to bright light (or normal light). Cones also cause the sensation of colour of objects in our eyes. Thus, cones present in the retina of the eye are responsible for colour vision. Cones, however, do not function in dim light.
Blind Spot
At the junction of optic nerve and retina in the eye, there are no light sensitive cells (no rods or cones) due to which no vision is possible at that spot. This is called blind spot. Thus, blind spot is a small area of the retina insensitive to light where the optic nerve leaves the eye (see Figure). When the image of an object is formed at the blind spot in the eye, it cannot be seen by the eye. Blind spot is not sensitive to light because there are no light-sensitive cells like rods or cones in this region. The existence of blind spot in the eye can be demonstrated as follows.
Activity 4
- Take a drawing sheet and draw a thick cross and a circular dot on this sheet of paper about 7 cm apart (as shown in Figure).
- Hold the sheet of paper at an arm’s length from your eyes.
- Now close your left eye and look at the cross on the paper with your right eye. You will also see the black dot.
- Bring the sheet of paper towards you slowly.
- At a certain distance, the dot disappears (because its image has fallen on the blind spot of your right eye).
The disappearance of dot from view shows that there is a point on the retina of the eye which cannot send message to the brain when the image of dot falls on it. This point is the blind spot. The black dot disappears and cannot be seen by the eye because its image falls on the blind spot of the right eye.
You can repeat the above activity by closing the right eye, keep looking at the dot mark with left eye, and bring the sheet of paper from arm’s length towards you slowly. At a certain distance, the cross disappears (because its image falls on the blind spot of your left eye).
We can also use the cross and dot printed on this page of the book to show the existence of blind spot in the eyes. All that we have to do is to hold this book at arm’s length and bring it slowly towards us in the way described above.
Persistence of Vision
If we look at a bright object and then close our eyes, the image of the object will remain in our eyes for a very short duration. Thus, the image formed on the retina of the eye does not fade away immediately. The image of an object seen by our eyes persists (or remains) on the retina for about \(\frac{1}{16}\) th of a second even after the object has disappeared from our view. This happens because the stimulated light-sensitive cells of the retina take a little time to come back to their original state.
The ability of an eye to continue to see the image of an object for a very short duration even after the object has disappeared from view, is called persistence of vision. It is due to the phenomenon of persistence of vision that we are able to see movie pictures in a cinema hall. This is because if the still pictures of a moving object are flashed on our eyes at a rate faster than 16 pictures per second then (due to persistence of vision), the eyes perceive this object as moving. This will become more clear from the following discussion.
The movies that we see are actually a number of still pictures (in the form of a long film) which are taken in proper sequence with a movie camera. These still pictures are projected on the screen of a cinema hall at the rate of 24 pictures per second (which is faster than 16 pictures per second) with the help of a film projector. Under these conditions, the image of one picture seen on the screen persists on the retina till the image of next picture falls on the screen, and so on. Due to this, the slightly different images of the successive pictures present on the film merge smoothly into one another and give us the feeling of moving images (or movie). In this way we are able to see the moving images (or moving pictures) of actors and actresses on a cinema screen.
Activity 5
We can demonstrate the phenomenon of persistence of vision by performing an activity as follows. Take a piece of cardboard and a pencil [see Figure (a)]. Make the sketch of a bird on one side of the cardboard and the sketch of a cage on its back side [see Figures (b) and (c)]. Make two small, horizontal slits (or cuts) in the piece of cardboard, one near its top and the other near its bottom. Insert the pencil through the two slits on the cardboard as shown in Figure (b). Hold the lower end
of pencil between the palms of your hands. Move your palms forwards and backwards quickly so as to spin the cardboard rapidly. Look at the cardboard when it is spinning rapidly. We will find that when the cardboard spins rapidly, then bird is seen to be in the cage [see Figure (d)]. This happens due to the phenomenon of persistence of vision. When the cardboard is spinning rapidly, the image of bird persists on our retina for a very short while during which the image of cage also falls on the retina, making the bird appear to be in the cage.
Range of Vision of a Normal Human Eye
The farthest point from the eye at which an object can be seen clearly is known as the “far point” of the eye. The far point of a normal human eye is at infinity. This means that the far point of a normal human eye is at a very large distance. The nearest point up to which the eye can see an object clearly without any strain, is called the “near point” of the eye. The near point of a normal human eye is at a distance of 25 centimetres from the eye. The near point of an eye is also known by another name as the least distance of distinct vision.
This means that the least distance of distinct vision for a normal human eye is about 25 centimetres. In other words, the most comfortable distance at which one can read a book with normal eyes is about 25 centimetres from the eyes. The minimum distance at which the eye can see objects distinctly (or clearly), however, varies with age. From the above discussion we conclude that the range of vision of a normal human eye is from infinity to about 25 centimetres. That is, a normal human eye can see the objects clearly which are lying anywhere between infinity to about 25 centimetres.
A normal eye can see the distant objects as well as the nearby objects by focussing the images of distant objects as well as nearby objects on its retina by changing the thickness (or converging power) of its lens. The ciliary muscles of the eye can change the thickness of the soft and flexible eye-lens and hence change the converging power of eye-lens. A thin eye-lens has less converging power whereas a thick eye-lens has greater converging power on the rays of light coming from an object.
(i) When the eye is looking at a distant object, the ciliary muscles are relaxed due to which the eye-lens is thin (or less convex). The thin eye-lens has smaller converging power which is sufficient to converge the parallel rays of light coming from a distant object to form its image on the retina.
(ii) When the same eye is looking at a nearby object, the ciliary muscles get stretched due to which the eye-lens becomes thick (or more convex). The thick eye-lens has greater converging power which is required to converge the diverging rays of light coming from a nearby object to form its image on the retina.
Defects of Vision
We will now discuss three common defects of the eye (or defects of vision) called myopia, hypermetropia and cataract, very briefly.
(i) Some persons cannot see the distant objects clearly (though they can see the nearby objects clearly). They are said to have a defect of eye called myopia. Myopia is that defect of eye (or defect of vision) due to which a person cannot see the distant objects clearly (though he can see the nearby objects clearly). Myopia is caused either due to high converging power of eye-lens or due to eye-ball being too long. In an eye suffering from myopia, the image of distant object is formed in front of retina due to which the person cannot see it clearly. Myopia is corrected by using spectacles containing concave lenses (diverging lenses) of suitable power. The image of distant object is then formed on the retina and hence can be seen clearly.
(ii) Some persons cannot see the nearby objects clearly (though they can see the distant objects clearly). They are said to have the defect of eye called hypermetropia. Hypermetropia is that defect of eye (or defect of vision) due to which a person cannot see the nearby objects clearly (though he can see the distant objects clearly). Hypermetropia is caused either due to low converging power of eye- lens or due to eye-ball being too short. In an eye suffering from hypermetropia, the image of object is formed behind the retina due to which the person cannot see it clearly. Hypermetropia is corrected by using spectacles containing convex lenses (converging lenses) of suitable power. The image of nearby object is then formed on the retina and hence the person can see it clearly.
(iii) A yet another defect of the eye which usually comes in old age is cataract. The medical condition in which the lens of eye of a person becomes progressively cloudy resulting in blurred vision is called cataract. Cataract develops when the eye-lens of a person becomes cloudy (or even opaque) due to the formation of a membrane over it. Cataract decreases the vision of the eye gradually. It can even lead to total loss of vision of the eye. The vision of the person can be restored after getting surgery done on the eye having cataract. The opaque lens is removed from the eye of the person by surgical operation and a new artificial lens is inserted in its place. The eye-defect called cataract cannot be corrected by any type of spectacle lenses.
Care of the Eyes
Our eyes are a wonderful gift of nature. We must take proper care of our eyes and protect them from any kind of injury or damage so that they remain good throughout our whole life. Some of the precautions to protect our eyes and maintain healthy eyesight are as follows :
1. Wash your eyes at least twice a day with clean water.
2. Too little light or too much light is bad for eyes. Too little light causes eyestrain and headache. So, do not read or write in dim light because it puts strain on the eyes and may cause headache. Too much light (like that from viewing the sun directly, electric welding or laser torch) can injure the retina and damage the eyes. So, we should never look directly at the sun or other powerful lights. We should protect our eyes from the glare of bright light.
3. Always read by keeping the book at normal distance for distinct vision. Do not read by bringing the book too close to your eyes or by keeping it too far from the eyes.
4. Raise your eyes from time to time while reading, writing or watching television so as to relax the eyes.
5. Protect your eyes from injuries and foreign bodies (like dust particles and insects, etc). If something (like a dust particle or insect, etc.) gets into the eyes, splash the eyes with a lot of clean water. Do not rub the eyes with hands to prevent injury to the eyes. If need be, consult a doctor for treatment.
6. If you find difficulty in reading a book or writing on the blackboard or you have to squeeze your eyes to see clearly, get your eyes checked by an eye-specialist doctor immediately. And if doctor recommends, wear spectacles to regain normal eyesight.
7. In case of an injury or any other problem to the eyes, consult an eye-specialist doctor. Self-treatment can be dangerous to the eyes.
8. Vitamin A is essential for keeping the eyes healthy. Deficiency of vitamin A in the diet is responsible for many ailments of the eye including night blindness. The inability of eyes to see properly in dim light (especially at night) is called night blindness. Night blindness is an eye disease which is caused by the lack (or deficiency) of vitamin A in the diet of a person. We should also include those food items in our diet which contain vitamin A to prevent night blindness and keep our eyes healthy. The food items which are rich in vitamin A are : Carrots, Cod-liver oil (Fish liver oil), Green vegetables (such as Spinach), Cabbage, Broccoli, Eggs, Milk, Butter, Curd, Cheese and fruits such as Papaya and Mango.
Eyes of Other Animals
Animals have eyes of different shapes and sizes. For example :
(i) The eyes of a crab are quite small but they enable the crab to look all around. Due to this a crab can see an enemy even if it comes from behind.
(ii) Butterflies have large eyes which appear to be made up of thousands of little eyes. The eyes of a butterfly enable it to see not only in the front and sides but at the back as well.
(iii) A night bird (owl) can see very well in the night (in dim light) but not during the day (when there is bright light). This is due to the following reasons :
(a) The owl has big eyes having large cornea and a large pupil to allow more light in the eye (see Figure).
(b) The owl’s eyes have a large number of rods in the retina (which are sensitive to dim light) and only a few cones (which are sensitive to bright light).
(iv) The day light birds (such as kite and eagle) can see very well during the day (in bright light) but not in the night (when there is dim light). The eyes of day light birds (such as kite and eagle ) are small and have more cones in retina (which are sensitive to bright light) but fewer rods (which are sensitive to dim light).
Visually Challenged Persons Can Read and Write
Some persons may lose their eyesight due to an eye injury (received during an accident) or due to a disease. And some persons cannot see at all from birth. Those persons who are ‘unable to see’ are known as visually challenged persons. Such persons develop their other senses more sharply. They try to identify things by touching and by listening to voices more carefully.
The most popular resource for visually challenged persons which can make them read and write is braille. Braille is a written language for the visually challenged persons in which characters (letters and numbers, etc.) are represented by patterns of raised dots. The visually challenged persons recognise the words written on a braille sheet by touching the patterns of dots (which are raised slightly to make it easier to touch).
The braille was developed by Louis Braille who was himself a visually challenged person. There is a braille code for common languages, mathematics and scientific notations. Many Indian languages can be read and written by using the braille system. Braille writer slate and stylus help the visually challenged persons in taking notes, reading and writing. Type-writer like devices for braille are also available now. Auditory aids such as cassettes, tape recorders, talking books and talking calculators are also very useful for such persons.
There are many visually challenged persons in the world who have great achievements to their credit. Helen Keller, an American lecturer and author is perhaps the most famous and inspiring visually challenged person of the world. Helen Keller lost her eyesight when she was only 18 months old. But because of her resolve and courage she studied up to graduation in a University and wrote a number of books, including ‘The Story of My Life’. Some visually challenged Indians have also made many achievements. For example, Ravindra Jain, who is visually challenged by birth, obtained the Sangeet Prabhakar degree from Allahabad University. He is a famous lyricist, singer and music composer. A visually challenged boy Diwakar is a very good singer.