The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.
CONSTRUCTION OF THE EYE
Cornea
The front part of the eye is called cornea. It is made of a transparent substance.
The outer surface of cornea is convex in shape.
Most of the refraction (bending) for the light rays entering the eye occurs at the outer surface of the cornea. It acts as a window for the human eye and its role is to prevent foreign substances from entering the eye
Iris
Iris is present just behind the cornea of the eye.
It is a flat, coloured and, ring-shaped membrane
It is a dark muscular diaphragm thatcontrols the size of the pupil(aperture).
It is like a camera shutter that controls the amount of light entering the eye.
Pupil
Pupil is a hole present in the middle of the iris.
It regulates and controls the amount of light entering the eye.
Eye lens
Eye lens is a convex lens that focuses the light rays entering the eye like an autofocus lens function in a camera. This is called accommodating power of the eye.
It is made up of transparent, soft and flexible material like a jelly made of proteins is present, located behind the pupil of the eye.
The eye- lens changes its shape to thick or thin when retina is exposed to the light. Hence, they are flexible.
Ciliary muscles
Its function is to maintain the spherical nat ure of the eye.
Vitreous Humor
Its function is to maintain the spherical nat ure of the eye.
It is a jelly like substance that is present almost all over the inner chamber of the eye.
After passing through the vitreous humor the light rays strike the retina.
Retina
The retina is located behind the eye-lens and at the back part of the eye.
It is the innermost lining of the inner tissue which consists of the sclera and the choroid.
The retina is a delicate membrane having a large number of light sensitive cells called ‘rods’ and ‘cones’
Retina responds to the ‘intensity of light’ and ‘colour of objects’ respectively, by generating electrical signals. Thus, it is light-sensitive.
When light finally strikes the retina, an electric signal is sent to the brain (the visual cortex) via the optic nerve. The brain then processes the image and finally we see what we see.
Blind spot
Blind spot is located at the junction of optic nerve and retina in the eye.
There are no light sensitive cells.
Blind spot is a small area of the retina insensitive to light where the optic nerve leaves the eye
When the image of an object falls on the blind spot, it cannot be seen by the eye.
FUNCTION OF IRIS AND PUPIL
The iris controls the amount of light entering the eyes.
If the amount of light received by the eye is large, as during the day time, then the iris contracts the pupil (makes the pupil small) and reduces the amount of light entering the eye.
Similarly, if the amount of light received by the eye is small as in a dark room or during night, the iris expands the pupil (makes the pupil large) so that more light may enter the eyes.
ADVANTAGES OF HAVING A PAIR OF EYES
On an average, a normal eye has a field of 170o with stereoscopic vision.
Another advantage is that an optimum level of brightness is provided in the surroundings.
Double eye vision reduces parallax error and increases depth of perception so that even faint objects are distinctly visible.
