Sense organs are groups of receptor cells, each tuned to one kind of stimulus. The eye is the classic example: the cornea and lens bend light on to the retina, the iris adjusts how much gets in, and the optic nerve carries the impulses to the brain. How the iris muscles work, how the lens changes shape, and the rods, cones and fovea are Extended content.
Sense organs are groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals. In the eye the cornea refracts light, the iris controls how much light enters the pupil, the lens focuses light on to the retina, the retina contains light receptors (some sensitive to different colours) and the optic nerve carries impulses to the brain. The pupil reflex changes pupil diameter as light intensity changes.
Click each part of the cut-away eye (or its chip) to read what it does. Then switch to the rods and cones view (Extended) and look at the graph of receptors across the retina.
| Part | Function |
|---|---|
| Cornea | refracts (bends) light; it does most of the focusing |
| Iris | controls how much light enters the pupil |
| Pupil | the hole in the iris through which light enters |
| Lens | focuses light on to the retina |
| Retina | contains light receptors, some sensitive to light of different colours |
| Optic nerve | carries impulses from the retina to the brain |
| Blind spot | where the optic nerve leaves the eye; no light receptors, so no image is detected there |
The blind spot and the fovea are different places. The fovea has the most receptors (cones); the blind spot has none at all, because the optic nerve leaves there.
Drag the light slider from dim to bright and watch the iris. The panel reads the pupil diameter, from about 8 mm in dim light to about 2 mm in bright light, and which muscles are contracted.
| Bright light | Dim light | |
|---|---|---|
| Pupil diameter | decreases (pupil constricts) | increases (pupil dilates) |
| Light entering | less, protecting the retina from damage | more, so you can still see |
| Circular muscles (Extended) | contract | relax |
| Radial muscles (Extended) | relax | contract |
Circular muscles contracting make the pupil smaller, like pulling a drawstring. Students often swap the two sets: radial muscles contract to make the pupil wider.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the eye tab, what does the cornea do?
Why is no image detected on the blind spot?
In the pupil reflex tab, what happens to the pupil as the light is made bright?
Extended: in the accommodation tab, what is the state of the ciliary muscles when the object is 25 cm away?
Extended. Move the object from far away to 25 cm and watch the ciliary muscles, the suspensory ligaments and the lens. The panel reports the state of each.
| Near object | Distant object | |
|---|---|---|
| Ciliary muscles | contract | relax |
| Suspensory ligaments | slacken (less tension) | pulled tight (more tension) |
| Lens shape | fatter, more convex | thinner, less convex |
| Refraction of light | more | less |
When the ciliary muscles contract, the ligaments go slack, not tight. The ciliary ring gets smaller, so it stops pulling on the lens, and the elastic lens springs fatter.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which part of the eye contains light receptors, some sensitive to light of different colours?
Extended: a person walks from a dark room into sunshine. Which change happens in the iris?
Extended: a student looks up from a book to a distant tree. What happens?
Extended: why do we see colours best when looking straight at an object?
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