5.2.3: The eye (biology only)
Not started yet — this one needs some love.
The eye is a sense organ with receptors sensitive to light intensity and colour.
The retina contains light receptors and the optic nerve carries impulses from the retina to the brain.
The sclera supports and protects the eye, and the cornea refracts light entering the eye.
The iris controls pupil size to change how much light enters the eye.
Accommodation is changing the shape of the lens to focus on near or distant objects.
For a near object, ciliary muscles contract, suspensory ligaments loosen, and the lens becomes thicker so it refracts light strongly.
For a distant object, ciliary muscles relax, suspensory ligaments are pulled tight, and the lens is pulled thin so it refracts light only slightly.
Myopia and hyperopia occur when light rays do not focus on the retina; spectacle lenses refract the rays so they do focus on the retina.
Other treatments include hard and soft contact lenses, laser surgery to change the cornea and replacement lenses in the eye.
Common exam mistakes
For near vision, do not say the lens becomes thinner; it becomes thicker and refracts light more strongly.
For distant vision, do not say the ciliary muscles contract; they relax and the suspensory ligaments are pulled tight.
Do not confuse the iris with the lens; the iris controls pupil size, while the lens changes shape for focusing.
In myopia and hyperopia questions, state where rays focus relative to the retina and how the correcting lens changes their path.
Ray diagrams need the corrected rays to focus on the retina, not just pass through a spectacle lens.