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Answer:
1........ human eye is the most valuable sense organ of the human body.
IT CONSISTS OF FOLLOWING PARTS:
Cornea : it is thin membrane covering the outer surface of the eye ball through which the light enters into the eye . it acts as a primary lens .
Iris: it is dark muscular diaphragm located just behind the cornea . it controls the size of the pupil.
Pupil : it is a hole in the iris, it regulates and controls the amount of light entering the eye.
Ciliary muscle : these muscles hold the eye lens in its position , they also help in changing the focal lenght of the eye lens to focus the image on the retina
eye lens / crystalline lens : it is composed of jelly like proteineous substance . it is convex in nature , it focuses the incoming light rays from the object on to the retina
Retina : it is a delicate membrane having large number of light sensitive cell called rods and cones , it acts like a screen . a real , inverted and diminished image is formed by the eye lens on it .
Optic nerve : the electrical signals generated by the light sensitive cells are carried to the brain by the optic nerve.
Blind spot : a small region of the retina where the optic nerve enters the eye ball is insensitive to light is called the blind spot.
2...........Myopia : in this defect a person cann see near by objects clearly but cannot see distant objects clearly.this defect can be corrected by using concave lens of suitable focal lenght
Hypermetropia : in this defect a person cannot see near by objects clearly . this defect can be corrected by using convex lens of suitable focal lenght.
Presbyopia : in this defect the person finds it difficult to see near by objects comfortably and clearly. this defect can be corrected by using convex lens of suitable focal lenght.
3............the ability of an eye to focus the distant objects as well as near by objects on the retina by changing the focal lenght of the eye lens is called power of accomodation.
4..............the medical condition in which the eye lens of a person becomes progressively cloudy which results in blur vision is known as cataract. the vision of such a person can be restored by a surgery.
5.............the minimum distance at which the object must be placed so that a person can see it clearly without any strain is known as near point of clear vision.
6..........refraction through a glasx bar is due to change in speed of light from one medium to anothet but in glass prism it occurs due to varying wave lenght of different colours of a ray of light.
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