The Eye

The EyeThe conjunctiva, lubricates the and protects the surface of the eye.Light is refracted into the eye by the cornea, it is transparent, has no blood vessels, so diffuses its oxygen from its outer surface.The iris controls the diameter of the pupil, thus controlling how much light enters the eye.The lens focuses light onto the retina at the back of the eye, which is sensitive to light, it is covered in sensors called rods and cones.

Rods are more sensitive to dim light, but cannot see colour.Cones are sensitive to colours but are not good in dim light.Cones are found all over the retina, but have a large concentration at the Fovea.The optic nerve carries the impulses from the receptors to the brain.

The Focusing Eye.By changing the shape of the lens or accommodation the eye can focus light.When looking at distant objects the ciliary muscles relax, which allows suspensory ligaments to pull tight.  The thins out the lens, making it less curved.Conversely when looking at near objects the ciliary muscles contract, which slackens the suspensory ligaments.  The lens becomes fatter, more curved.

Short sightedBeing short sighted makes seeing distant objects difficult.The cornea or lens bends light too much or if the eyeball is to long, it will bring the image into focus in front of the retina.A corrective lens will help to focus the light onto the retina again.

Long SightedBeing long sighted makes it difficult to focus on near objectsThis happens when the cornea or lens does not bend enough or the eyeball is too short.This will cause the image to brought into focus behind the retina.As with short sight, a corrective lens can be used to focus the light correctly onto the retina.

The Iris ReflexAs with similar reflex arc's that help to reduce injury to ourselves, the eye has one too.Very bright light can damage the retina, so the eye has a reflex to protect it.Very bright light triggers a response that allows the pupil to become smaller and let less light in.Conversely in dim light the response is the opposite, the pupil will get bigger as the brain tells the radial muscles to contract.

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