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Unlock This CourseYou already know the parts of the eye from the lesson Structure and Function of the Eye. Now we look at how those parts move. The eye has to do two things all day long. It must focus light from objects at different distances, and it must cope with light that is sometimes dazzling and sometimes dim. Both jobs are done by muscles that change the shape of the lens or the size of the pupil.
Key terms:
Light from an object must be focused exactly onto the retina to make a sharp image. The cornea does some of the bending. The lens does the fine adjustment, and it can do this because it is flexible. The ciliary muscles and the suspensory ligaments change its shape.
Light from a close object spreads out a lot, so it needs bending strongly. The ciliary muscles contract. This makes the ring of muscle smaller, so the suspensory ligaments slacken. The lens is no longer pulled tight, so it becomes thicker and more curved. A fatter lens bends the light more.
Light from a faraway object is almost parallel, so it needs bending less. The ciliary muscles relax. The ring of muscle gets wider, so the suspensory ligaments become tight and pull on the lens. The lens becomes thinner and less curved. A thin lens bends the light less.
| Near object | Distant object | |
|---|---|---|
| Ciliary muscles | Contract | Relax |
| Suspensory ligaments | Slacken (loose) | Tight |
| Shape of lens | Thicker, more curved | Thinner, less curved |
| Bending of light | More | Less |
A good way to remember it: contract the muscles to look at a close object, like holding a book. Relaxing the muscles is the restful position for gazing into the distance. This is why your eyes can feel tired after a long time reading or looking at a phone.
Too much light can damage the retina. Too little light and you cannot see detail. The iris controls how much light enters through the pupil. It has two sets of muscles that work against each other: circular muscles and radial muscles.
The circular muscles contract and the radial muscles relax. The pupil gets smaller. Less light enters, which protects the retina from damage.
The radial muscles contract and the circular muscles relax. The pupil gets larger. More light enters, so you can see better.
| Bright light | Dim light | |
|---|---|---|
| Circular muscles | Contract | Relax |
| Radial muscles | Relax | Contract |
| Size of pupil | Smaller | Larger |
| Amount of light entering | Less | More |
The change in pupil size is a reflex. It happens automatically, without you thinking about it, just like the reflexes in the lesson The Simple Reflex Arc. Next time you leave a dark cinema into bright sunshine, your pupils shrink within a moment or two.
Near object: ciliary muscles contract, ligaments slacken, lens thick.
Distant object: ciliary muscles relax, ligaments tight, lens thin.
Bright light: circular muscles contract, pupil small.
Dim light: radial muscles contract, pupil large.
(a) State what happens to the shape of the lens when a person changes from looking at a distant tree to reading a book. [1 mark]
(b) Describe how the ciliary muscles and suspensory ligaments cause this change. [2 marks]
(c) Explain how the iris reduces the harm to the eye when a person walks out into bright sunshine. [3 marks]
(a) The lens becomes thicker (more curved) (1)
(b) The ciliary muscles contract (1), which makes the suspensory ligaments slacken so the lens is not pulled tight (1)
(c) The circular muscles of the iris contract (1) and the radial muscles relax (1), so the pupil gets smaller and less light reaches the retina, protecting it from damage (1)
In "explain" questions name the muscles and say whether they contract or relax. "The pupil changes size" is not enough: always say which way, and what that does to the amount of light.