The term vision includes several functions like:

• Visual acuity
• Visual fields

In addition, color vision, dark adaptation and contrast sensitivity are very important elements. Vision is dependent upon light stimulating the sensitive cells of retina, but along the route through different media several things happen: reflection, transmission and absorption.

The velocity of light depends on the amount of resistance (optical density) offered by the medium. When a lightbeam enters a medium of greater resistance, the velocity of the beam will be retarded. If the lightbeam hits the other medium obliquely, this will result in a change of direction known as refraction.

Refraction of the eye and optics are fundamental subjects in vision and eye care.