Construction of human eye The outermost black part of the eye is called cornea. The iris is present behind it. The hole present in the iris is called pupil. Behind this a crystalline lens is present which is held by ciliary muscles. The retina is the innermost part of the eye which is connected by the optic nerves. The liquid present between the iris and the eye lens is called aqueous humour and the liquid present between the eye lens and the retina is called vitreous humour.
Functions of
Working of eye The light falling on the eye gets refracted through the cornea. This amount of light is controlled by the pupil. The eye lens converges the light rays and forms an image on the retina. The focal length of the lens is controlled by the ciliary muscles. Retina converts the light rays into electrical signals which are sent by optic nerves to the brain. The brain interprets these signals and we see the objects as it is.
Power of accommodation. The ability of an eye to focus the distant objects as well as nearby objects on the retina clearly by changing the focal length of lens is called power of accommodation. A normal eye has a power of accommodation which enables the objects as far as infinity and as close as 25 cm to be focused on retina.
Eye lens. An eye lens is composed of fibrous jelly like material. Its focal length can be changed by ciliary muscles by pressing or stretching it. To increase the focal length, the ciliary muscles are in the relaxed position and the lens is thin.
Near point or least distance of distinct vision. The minimum distance at which an object placed can be seen clearly without strain is called near point or least distance of distinct vision. For a healthy human eye, near point is 25 cm.
Far point The farthest point up to which the eye can see the object clearly is called the far point of the eye. The far point for a healthy human eye is at infinity.
Cataract The condition in which the lens of the eye of a person becomes cloudy and milky resulting in blurred vision is called cataract.
Cause of formation of cataract Cataract develops when the eye lens of a person becomes cloudy due to the formation of a membrane of proteins over it.
Reason of having two eyes for vision.
Defects of vision
Myopia or near sightedness short sightedness The defect of vision in which a person can see the nearby objects clearly but is unable to see the distant objects clearly is called short sightedness or myopia. In the myopic eye the image of the distant object is formed in front of the retina.
