The eye is cut in half so you can see inside. Press Play to follow light from the cornea to the brain, click any structure, or pick an eye condition to see where the focus goes wrong and how glasses fix it.
Seeing begins with light bouncing off objects into your eye. The eye works much like a camera: a clear front element gathers and bends the light, an adjustable opening controls how much gets in, a focusing element sharpens the image, and a light-sensitive surface at the back records it. Each of these has a name — cornea, pupil, lens, retina — and the whole job is to land a sharp image on the back of the eye.
Light bends (refracts) whenever it crosses between materials of different density. The biggest bend happens right at the front, where light goes from air into the curved cornea — this provides about $\tfrac{2}{3}$ of the eye's focusing power. The coloured iris then opens or closes the pupil to control brightness, just like a camera aperture. Behind it the lens adds the final third of focusing and, crucially, changes shape to fine-tune the focus. The goal is to make all the rays from one point meet again at one point on the retina.
Accommodation is the lens reshaping: ciliary muscles contract to make it rounder for near objects and relax to flatten it for far ones. Because a converging lens makes rays cross, the image on the retina is upside-down and reversed — your brain has simply learned to read it as upright. At the retina, two photoreceptors fire: about $120$ million rods for dim, colourless, peripheral vision and about $6$ million cones for bright, colour, detailed vision, packed tightest at the fovea. Their signals leave through the optic nerve; where it exits there are no receptors — the blind spot.
Press Play to send light from the cornea to the optic nerve. Slide the Object distance from far away to 25 cm and watch the lens round up while the Accommodation readout climbs — the rays still meet on the retina. Now pick Myopia: the rays cross in front of the retina and the eye chart blurs; switch on the Corrective lens and a concave lens moves the focus back. Try Presbyopia with a book at 25 cm, then shrink the pupil with Bright light — a small pupil makes the blur smaller, which is why people squint. The blue rays from the top of the object land below the axis: the image really is upside-down.
| Stage | Structure | What it does |
|---|---|---|
| 1. Enter | Cornea | Clear front dome; bends light most (~⅔ of focusing power) |
| 2. Control light | Iris & pupil | Iris opens/closes the pupil to set how much light enters |
| 3. Fine-focus | Lens | Changes shape (accommodation) to focus near or far |
| 4. Image forms | Retina | Rays converge to an inverted image at the back of the eye |
| 5. Detect | Rods & cones | Photoreceptors convert light into electrical nerve signals |
| 6. Transmit | Optic nerve | Carries the signals to the brain, which interprets the image |
| Part | What it is | Job |
|---|---|---|
| Cornea | Clear curved front window | Does about two-thirds of the focusing |
| Aqueous humour | Watery fluid behind the cornea | Feeds the cornea and lens; keeps the eye’s pressure |
| Iris | Coloured ring of muscle | Changes the pupil size to control light |
| Pupil | Hole in the iris | Lets light in |
| Lens | Clear, flexible disc | Fine-focuses by changing shape (accommodation) |
| Ciliary body & muscle | Muscular ring around the lens | Contracts to let the lens round up for near vision |
| Suspensory ligaments | Fibres holding the lens | Pull the lens flat for distance vision |
| Vitreous humour | Clear jelly | Fills and shapes the eyeball |
| Retina | Light-sensitive inner layer | Rods and cones turn light into nerve signals |
| Macula & fovea | Central spot and pit of the retina | Sharpest, colour vision |
| Optic disc | Where the nerve leaves | No receptors — the blind spot |
| Choroid | Dark, blood-rich layer | Feeds the retina; absorbs stray light |
| Sclera | Tough white outer coat | Protects the eye and holds its shape |
| Optic nerve | Cable of about 1.2 million fibres | Carries signals to the brain |
| Condition | Cause | Blurred | Focus falls | Correction |
|---|---|---|---|---|
| Myopia (short-sight) | Eyeball too long or cornea too curved | Far objects | In front of the retina | Concave (minus) lens |
| Hyperopia (long-sight) | Eyeball too short or too little focusing power | Near objects (and far, if severe) | Behind the retina | Convex (plus) lens |
| Presbyopia | Lens stiffens with age; cannot round up | Near objects, from about age 45 | Behind the retina (near objects) | Convex reading lens or varifocals |
| Astigmatism | Cornea curved more in one direction than the other | Near and far; lines in one direction | Two different places | Cylindrical (toric) lens |
| Cataract | Lens becomes cloudy | Everything hazy, dim, glare | Scattered | Surgery: artificial lens implant |
A lens’s strength is measured in dioptres (D): power = 1 / focal length in metres. A −3.00 D glasses lens is a diverging lens with a focal length of about 33 cm; the minus sign means concave. About 1 mm of extra eyeball length gives roughly 2.5–3 D of myopia.
Light bends most where the change in optical density is greatest. Going from air ($n\approx 1.00$) into the cornea ($n\approx 1.38$) is a big jump, so the cornea bends light strongly and supplies roughly two-thirds of the eye's total power (about $40$ of $\sim 60$ dioptres). The lens sits surrounded by watery fluids of similar density, so each surface bends light less — but because the lens can change shape, it provides the adjustable focusing the fixed cornea cannot. This is also why laser eye surgery reshapes the cornea: small changes there have a large effect on focus.
When light reaches a rod or cone, it strikes a pigment (rhodopsin in rods) and changes its shape, triggering a cascade that alters the cell's electrical state. Signals pass to bipolar and ganglion cells, whose long fibres gather into the optic nerve. The fovea, a tiny pit packed almost entirely with cones, gives the sharp central vision you use for reading; move text just a few degrees to the side and it blurs, because the periphery is rod-dominated. The point where the optic nerve and blood vessels leave the eye, the optic disc, has no receptors at all — your personal blind spot.
Light enters through the clear cornea at the front, passes through the watery aqueous humour, then through the pupil — the opening in the coloured iris. It continues through the lens, crosses the jelly-like vitreous humour that fills the eyeball, and finally lands on the retina at the back. There the photoreceptors turn light into nerve signals that travel down the optic nerve to the brain.
Key takeaway: light goes cornea → pupil → lens → retina, and the retina sends signals through the optic nerve to the brain.Most people are surprised that the cornea does about two-thirds of the eye's focusing, because light bends most when it first crosses from air into the curved, transparent cornea. The lens provides the remaining third, but its job is the adjustable fine-tuning: it changes shape to focus on near or far objects. So the cornea is the main fixed lens, and the lens is the fine-focus dial.
Key takeaway: the cornea provides most of the fixed focusing power; the lens fine-tunes focus for different distances.Accommodation is how the eye changes focus between near and far objects by reshaping the lens. Ciliary muscles surround the lens: when they contract, the lens becomes rounder and thicker to focus on close objects; when they relax, the lens flattens to focus on distant ones. The ability to accommodate decreases with age (presbyopia), which is why many people need reading glasses after about $40$.
Key takeaway: accommodation is the lens changing shape — rounder for near, flatter for far — driven by the ciliary muscles.Because the cornea and lens bend converging rays so they cross before reaching the retina, the image that lands on the retina is inverted (upside-down) and reversed left-to-right. This is simple optics — any single converging lens does the same. Your brain has learned since birth to interpret this flipped image as right-way-up, so you never notice it.
Key takeaway: a converging lens flips the image, so the retina receives an upside-down picture that the brain interprets as upright.The retina has two kinds of photoreceptors. Rods are extremely sensitive and work in dim light, but they do not detect colour and give low-detail vision — they dominate the periphery and night vision. Cones need brighter light, detect colour (red, green and blue types), and give sharp detail; they are packed most densely at the fovea. There are about $120$ million rods and $6$ million cones in each eye.
Key takeaway: rods give sensitive, colourless night and peripheral vision; cones give sharp, colour, daytime vision and crowd the fovea.Clear vision needs the image to focus exactly on the retina. In nearsightedness (myopia) the eyeball is too long or the cornea too curved, so distant objects focus in front of the retina and look blurry; concave (diverging) lenses correct it. In farsightedness (hyperopia) the eyeball is too short, so the focus falls behind the retina; convex (converging) lenses correct it. Both are about where the focal point lands relative to the retina.
Key takeaway: myopia focuses light in front of the retina and hyperopia behind it; corrective lenses shift the focus back onto the retina.They add or remove focusing power in front of the eye so the image lands exactly on the retina. For myopia the eye focuses too strongly, so a concave (minus) lens spreads the light out slightly first; for hyperopia and presbyopia a convex (plus) lens adds the power the eye lacks. Contact lenses do the same job sitting on the cornea, and laser surgery does it by reshaping the cornea itself.
Key takeaway: concave lenses fix short-sight, convex lenses fix long-sight and presbyopia.The blind spot is the optic disc, where about a million nerve fibres and the retinal blood vessels leave the eye. There are no rods or cones there, so light landing on it cannot be seen. It lies about 15° to the side of where you are looking. We do not notice it because each eye covers the other’s blind spot and the brain fills in the gap using the surrounding pattern.
Key takeaway: the optic disc has no photoreceptors; the brain and the other eye hide the gap.A cataract is a clouding of the eye’s lens, most often caused by ageing, but also by diabetes, smoking, long exposure to sunlight or injury. Light is scattered and absorbed, so vision becomes hazy, dim and yellowish, with glare around lights. Glasses cannot fix it; the usual treatment is a short operation that removes the cloudy lens and replaces it with a clear plastic one. Worldwide, cataract is the leading cause of blindness.
Key takeaway: a cataract is a cloudy lens, treated by replacing the lens.