Vision Basic function of Eye

Anatomy of the Human eye  

1. Cornea  2. Iris and Pupil 3.  Lens  4. Retina

Cornea - is a clear ‘window’ at the very front of the eye. Cornea acts as a fixed focusing device. 

 Focusing is achieved by the shape of the cornea bending the incoming light rays.            Responsible for 70%-80% of the total   focusing ability (refraction) of the eye.

Iris and Pupil 

Iris ( coloured part of the eye) controls the amount of light that is allowed to enter the eye.      It does this by varying the size of the pupil (the dark area in the centre of the iris).              The size of the pupil can be changed very rapidly to cater for changing light levels. 
  The amount of light can be adjusted by a factor of 5:1.

After passing through the pupil, the light passes through the lens. 

Lens shape is changed by the muscles (cillary muscles) surrounding it which results in the final focusing adjustment to place a sharp image onto the retina. 

Accommodation -- Change of shape of the lens is called accommodation.                                Degree of accommodation can be affected by factors such as fatigue or the ageing process.                                                                                                                                                        In order to focus clearly on a near object, the lens is thickened.                                                  To focus on a distant point, the lens is flattened. 

 Retina -- is located on the rear wall of the eyeball.  It is made up of a complex layer of nerve cells connected to the optic nerve. 

Rods and Cones - Two types of light sensitive cells are found in the retina.
The central area of the retina is known as the fovea and the receptors in this area are all cones. It is here that the visual image is typically focused. 
Moving outwards, the cones become less dense and are progressively replaced by rods, so that in the periphery of the retina, there are only rods.

Factors Affecting Clarity of Sight-An individual with an acuity of 20/20 vision should be able to see at 20 feet that which the so-called ‘normal’ person is capable of seeing at this range. It  may be expressed in metres as 6/6 vision. The figures 20/40 mean that the observer can read at 20 feet what a ‘normal’ person can read at 40 feet.

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