HUMAN EYE:
Eye is one of our most important sense organs. The main parts of the human eye are:
Cornea, Iris, Pupil, Ciliary muscles, Eye lens, Retina, and Optic nerve.
1. Our eye is shaped like a ball. It has a roughly spherical structure.
2. Outer coat of eye is white.
3. The front part of the eye is called Cornea. 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.
4. Just behind the Cornea, there is Iris. Iris is the coloured part of the eye. The iris has a hole at its centre which is called pupil.
5. The eye lens is a convex lens which is behind the pupil.
6. The eye lens is held in position by ciliary muscles. It controls the eye lens.
7. The retina is a screen on which the image is formed in the eye. The eye lens focuses the image of an object on the retina.
8. The optic nerve carries the image formed on retina to the brain.
WORKING OF THE EYE
1. Light from the object enter Pupil of the eye and fall on the eye lens.
2. 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.
3. The retina has a large number of light sensitive cells.
4. When the image of the object falls on the retina, then the light sensitive cells generate electric signals.
5. The retina send this electrical signals to the brain through the optic nerve and we are able to see the object.
6. Although the image of an object formed on the retina is inverted but our brain interpret this image as that of an erect image.
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.
RODS AND CONES
Rods are the rod-shaped cells present in the retina of an eye which are sensitive to dim light.
Cones are the cone shaped cells present in the retina of an eye which are sensitive to bright light. Cones also cause the sensation of colour of objects in our eyes.
BLIND SPOT
Blind spot is a small area of the retina insensitive to light where the optic nerve leaves the eye.
PERSISTENCE OF VISION
The ability of an eye to continue to see the image of an object for a very short duration even after the image has disappeared from view is called persistence of vision.
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.
The nearest point upto which the eye can see an object clearly without any strain is called near point of the eye. The near point of a normal human eye is at a distance of 25 cm from the eye.
DEFECTS OF THE EYE
Myopia is the defect of eye due to which a person cannot see e the distant objects clearly though he can see e the nearby objects clearly.
Myopia is corrected by using spectacles containing concave lenses.
Hypermetropia is the defect of eye due to which a person cannot see the nearby objects clearly though he can see the distant objects clearly.
Hypermetropia is corrected by using spectacles containing convex lenses.
The medical condition in which the lens of eye of a person becomes progressively cloudy resulting in blurred vision is called cataract.
Cataract can be corrected with the help of surgery done on the eye.
CARE OF THE EYES
1. Wash our eyes at least twice a day with clean water.
2. We should not read or write in dim light.
3. We should not read by bringing the book too close to our eyes or too far from the eyes.
4. We should raise our eyes from time to time while reading, writing or watching television.
5. We should not rub the eyes with hands to prevent injury to the eyes.
6. In case of any problem we should consult an eye specialist.
7. We should take vitamin A regularly to keep our eyes healthy.
NIGHT BLINDNESS
The inability of eyes to see properly in dim light during night is called night blindness.
EYES OF OTHER ANIMALS
1. The eyes of a crab are quite small but they enable the crab to look all around.
2. Butterflies have large eyes which appear to be made up of thousands of little eyes. They can see all around.
3. Owl can see very well in the night, but not during the day.
VISUALLY CHALLENGED PERSONS CAN READ AND WRITE
Those persons who are unable to see are known as visually challenged persons.
Braille is a written language for the visually challenged persons in which characters like numbers and letters are represented by patterns of raised dots.
