Structure and function of the eye

Biology · Anatomy

tap any part to see its job

Switch the names off and test yourself: can you name every part before you tap it?

A side view of the eye, with light coming in from the left. Tap each part to find out what it does.

Follow the light

Step through what happens between seeing an object and your brain knowing it's there.

1. 1CorneaLight from the object reaches the cornea first. The cornea refracts it, bending the rays so they start to come together (converge).+
2. 2LensNext the light passes through the lens, which converges the rays further. The lens finishes the job the cornea started.+
3. 3Focal point on the retinaThe rays meet at a focal point on the retina. That's what creates a clear image.+
4. 4Receptors respondThe sensory receptors in the retina detect the light: how bright it is (its intensity) and what colour it is (its wavelength).+
5. 5Optic nerve to brainThe receptors pass electrical information along the optic nerve, made of sensory neurones, to the brain. The brain processes it and interprets the image.+

Opposite conditions, opposite settings

The iris has two sets of muscles: circular muscles and radial muscles. In bright light, the circular muscles contract and the radial muscles relax. The pupil gets smaller, so less light gets in, and that protects the retina from damage.

Now the lights go dim. What happens in the iris?

What you need to know

• The eye is a sense organ. Receptors in the retina detect light's brightness (intensity) and colour (wavelength), and the optic nerve carries that information to the brain.
• Learn nine structures and their jobs: cornea, iris, pupil, lens, suspensory ligaments, ciliary muscles, sclera, retina and optic nerve.
• Light is refracted twice. The cornea converges the rays slightly, then the lens converges them further so they meet on the retina. Only the lens changes shape.
• Two adjustments: the ciliary muscles change the shape of the lens so you can focus near or far, and the iris muscles change the size of the pupil for bright or dim light.

The big picture

The eye is a sense organ: receptors in the retina detect light, and the optic nerve carries the information to the brain. The cornea converges light slightly and the lens converges it further, so the rays meet on the retina. Only the lens changes shape. For a near object the ciliary muscles contract, the ligaments loosen and the lens thickens; for a distant object the reverse happens. In the iris, antagonistic circular and radial muscles make the pupil smaller in bright light and bigger in dim light, as a reflex.

⚠ Watch out

Thinking the ciliary muscles contract to pull the lens thin. It's the other way round. When the ciliary muscles contract, the suspensory ligaments loosen and the lens gets thicker, which is what you need for a near object.

Memory hook

Close up, Contract: when the ciliary muscles contract, the ligaments go slack and the lens goes thick. And for the iris, Circular muscles Close the pupil down in bright light.

Check yourself

Without looking back, sketch a side view of the eye and label all nine parts. Then add arrows to show the path light takes, and circle the one part that changes shape when you refocus.