The Eye

• The cross-section of the eye diagram is what the structure of the eye looks like sliced in half
• The cornea is completely transparent and contains no blood vessels
• All oxygen must diffuse into the cornea from the outside air
• The cornea refracts light
• The iris controls how big the pupil is
• The lens refracts light but it can also change its shape which will change how strongly it refracts light
• The lens focuses the light on the retina
• The retina is made up of cone cells which are sensitive to colour but don't work well in low light. There are also rod cells which are sensitive to light but see no colour.
• The fovea is a special sack of the retina that contains only cones. It is where the light is focused on.
• The optic nerve transmits all impulses generated by the receptor cells to the brain
• The iris has a reflex that reacts to light intensity. The more light intensity, the smaller the pupil will be.
• When the pupil is small its constricted
• When the pupil is large it is dilated.
• The iris is made of two types of muscles. On the inside circular muscles and on the outside radial muscles.
• To make the pupil smaller the circular muscles contract and the radial muscles relax.
• To dilate the pupil the circular muscles relax and the radial muscles contract.
• Accommodation is the reflex that changes the refractive power of the lens
• If an object is close, its rays must be refracted a lot to reach the fovea. This means the lens must be short and fat- it is more curved so it will refract more. The ciliary muscles will contract and the suspensory ligaments will slacken so this will let the lens be fatter.
• When an object is distant the rays don't need to be refracted as strongly. To reduce the refractive power of the lens, the ciliary muscles relax, and the suspensory ligaments become taut. This will stretch out the lens and reduce its refractive power.
• When the ciliary muscle contracts it moves inwards towards the lens
• The suspensory ligaments don't contract or relax. They are taut (contracted) or slackened (relaxed)
• When the lens can't refract enough someone is long-sighted. The light appears behind the retina which makes the image blurry. To fix this we use convex lens, these provide more refractive power.
• Short-sighted people refract light too much. The image is focused in front of the retina. To fix this we use concave lens which reduces refractive power and focuses the image on the retina.
• The medical term for long sight is hyperopia
• The medical term for short sight is myopia