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Visual Impairment :
Special Educational Needs
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Light entering the eye is controlled by the iris which is pigmented so that light can not pass through it.

Photo of human eyes

The size of the pupil depends on the movement of the iris.

In bright light the circular/sphincter muscles in the iris contract reducing the diameter of the pupil and in dim light the radial muscles contract opening. This protects the retina from damage that light of high intensity could cause.


The lens of the eye is transparent, bi-convex structure made of cells and is flexible. It changes shape to allow the lens to focus light entering the eye form objects at different distances. The adjustment of the lens is called accommodation.

Diagram showing how the muscles surrounding the lens change the focus

Distance vision

Lens adapted for distant objects
The ciliary muscles around the lens relax
The ligaments holding the lens become taut
The lens is pulled thin


Near vision

Lens adapted for close objects
The ciliary muscles around the lens cotract
The ligaments holding the lens slacken
The lens is allowed to shrink and become thicker



Light rays entering the eye from an object, such as the eye chart in the diagram, are focused onto the retina by the bi-convex lens. This inverts the image, horizontally and vertically, so it is upside down and back to front.

Diagram showing the way in which the lens focuses light rays onto the retina