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Human Eye

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Human Eye - Lesson Summary


Human eye is the most important organ of our body which is an optical device that serves as our organ of sight. It consists of a tough fibrous membrane called sclera that protects the internal parts of the eye.

Cornea is the membrane covering the front of the eye that is bulged out and is responsible for the maximum refraction of the light that enters the eye.

Aqueous humour lies behind cornea that enables the eye to cope up with the atmospheric changes.

Iris forms the coloured part of the eye. It adjusts the size of the pupil, thereby controls the amount of light entering the eye.

the crystalline lens, lies behind Iris a biconvex structure that helps in fine adjustment to the refracted light so that it is focused on the screen of the eye, which is referred to as retina.

Ciliary muscle helps in adjusting the focal length of the lens by contraction or relaxing. Vitreous humour lies behind the lens which is a dense, clear, jelly like fluid which helps to maintain the shape of the eye and focus the image clearly on the retina.

Retina is actually a canopy of the nerve endings of the optical nerve through which images are converted into electrical impulses and transferred to the brain for realization of the image.

The eye can focus near objects as well distant objects and this is accommodation of the eye. The minimum distance of the object at which an eye can focus clearly is the near point the maximum is called the far point.

If an eye is unable to focus the nearby objects and is able to view clearly the far off objects, the defect is called hypermetropia or long sightedness and can be corrected by a suitable convex lens.

On the other hand if a person is able to view the nearby objects clearly and unable to view the distant objects the defect is myopia or short sightedness and can be corrected by a suitable concave lens.

The power of the corrective lens is calculated by using lens formula and is measured in dioptre. The major parts of the human eye and their functions are as the following:

Functions of the Parts of Human Eye

Sclera
Protects and contains internal parts

Cornea
Cornea is responsible for maximum refraction of incident light

Aqueous Humour
Prevents collapse of the eye due to atmospheric pressure changes

Iris
Iris controls the light entering the eye by  adjusting the size of pupil

Crystalline Lens
Focuses the light reflected by objects on the retina.

Ciliary Muscles
Alter focal length of the crystalline lens

Retina
Receives optical image and converts it to electrical impulses

Vitreous Humour
Maintains the shape of the eye

Optic Nerve
Carries electrical impulses to the brain. Brain interprets these impulses and produces the sense of vision.

The functioning of a camera is similar to that of the eye.

Accommodation
The ability of the eye to change the focus between objects at different distances by altering the curvature of the lens is called accommodation.

        •  To form a clear image of different objects at different distances from the eye, the focal length of the eye lens has to be changed.
        •  The contraction and relaxation of ciliary muscles helps to alter the curvature of the lens.
        •  Far point is the maximum distance from the eye at which the eye can obtain a focused image of an object without straining.

Far point
The farthest point upto which a shortt sighted eye can see clearlyis called the farpoint of the eye. For a normal human eye, the far point is infinity.

Near Point
Near point is the minimum distance at which the eye can obtain a focused image of an object without straining. For a normal human eye, of an adultthe near point is about25 cm from the eye.

Least Distance of Distinct Vision
The minimum distance required between the object and the eye to view the object comfortably is called the least distance of distinct vision.
       
        •  Long exposure of the eyes to ultraviolet light, effects of diabetes, hypertension and old age can result in a condition called cataract.
        •  Cataract results in a cloudy translucent eye lens. This cloudiness affects the ability of the eye to accommodate.
        •  The horizontal field of view for a single eye is 150o.
        •  Stereovision position of the eyes, help us to see the maximum possible number of objects around us.

Factors causing irregularities in vision

        •  Irregularities on the surface of the cornea
        •  Development of cataract
        •  Weakening of ciliary muscles  
        •  Change in the size of the eyeball

These irregularities lead to the following three major types of defective vision.

        •  Myopia
        •  Hypermetropia
        •  Presbyopia  

Myopia
Myopia, also known as short sightedness or near sightedness, is a defect in which a human eye can see nearby objects clearly but distant objects appear blurred and unclear.
 
Myopia can be corrected by placing a suitable concave  lens in the line of sight.
 
Hypermetropia
Hypermetropia also known as long sightedness is a defect of vision in which a human eye has problems seeing objects located nearby, clearly.

        •  A person suffering from hypermetropia can see distant objects clearly.
        •  Hypermetropia occurs when the converging power of the eye lens is less than normal.
        •  Hypermetropia can be corrected by placing a suitable convex lens in the line of sight.

Presbyopia
Presbyopia is a condition in which the crystalline lens of an eye loses its flexibility.

Persons suffering from presbyopia are unable to read or see clearly even at the least distance of distinct vision, which is 25 centimetre. To correct presbyopia, a bifocal lens is used.


Correction of Hypermetropia and Myopia

To correct the short sightedness i.e. Myopia a concave lens whose focal length is equal to the distance of the far poin tof the myopic eye is to be placed in front of the the myopic eye.

To correct the Longt sightedness i.e. Myopia a converging i.e. convex lens of suitable focal length is used. When it is placed before the eye lens, the final image is focussed on to the retina.


Calculation of focal length of the corrective lens of  Longt sightedness (Myopia) 


The Focal length of the creective lens is calculated as the following.

If x is the distance of the near point of the defective eye and D is the least distance of distict vision, the 
u = - D,v  = -x, where u = Object distance and x = Distance of the near point of the defective eye
⇒(1/f) = [(1/v) -(1/u)]
⇒(1/f) = [-(1/x) =(1/D)]
⇒(1/f) = [(x D)/(x - D)]
⇒(1/f) = xD/(x - D)
Since x>D, the focal lenth is posive.



whose focal length is equal to the distance of the far poin tof the myopic eye is to be placed in front of the the myopic eye.

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