EYESIGHT – INTRODUCTION
Many people wear glasses and go regularly to an optometrist or an eye doctor.
Yet most are rather hazy about how their eyes function and why they need glasses.
Light enters through the cornea, or clear window, of the eye.
It passes through the watery fluid of the anterior chamber and enters the back section of the eye through the pupil, a hole in the iris or colored part of the eye.
The iris acts like the lens aperture of a camera, varying in size to admit different amounts of light depending on how bright it is.
This is a layer of sensitive nerve endings which when stimulated transmit the impulse through to the brain where it is interpreted as sight.
To test eyesight — or visual acuity — a Snellen’s chart is used.
This consists of letters which diminish in size from above downwards.
The top letter is of such a size that a person of normal sight should be able to see it at a distance of about 60 metres.
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