How do we see?
Seeing seems effortless, but it’s a remarkable partnership between your eyes and brain. Light enters your eye, is focused into an image, is turned into electrical signals, and is interpreted by your brain, all in a fraction of a second.
Parts of the eye
| Part | What it does | Camera comparison |
|---|---|---|
| Cornea | Clear front window; does most of the light bending | Front glass of the lens |
| Iris | Coloured muscle ring; controls pupil size | Aperture control |
| Pupil | Opening that lets light in | Aperture (hole) |
| Lens | Fine-tunes focus by changing shape | Adjustable lens |
| Ciliary muscles | Change the shape of the lens | Focus ring |
| Vitreous humour | Clear jelly that keeps the eye’s round shape | Inside of the camera body |
| Retina | Light-sensitive layer at the back | Image sensor |
| Optic nerve | Carries signals to the brain | Cable to the computer |
Step by step
- Light reflects off an object and enters the eye through the cornea.
- The pupil controls how much light gets in. In bright light, the iris shrinks the pupil; in dim light, it widens it.
- The cornea and lens bend (refract) the light, focusing it onto the retina.
- The image on the retina is upside down and reversed.
- Cells in the retina change light into electrical signals.
- The optic nerve carries the signals to the brain.
- The brain flips the image and interprets what you see.
Focusing: near and far
The lens changes shape to focus:
- Distant objects: ciliary muscles relax, and the lens gets thinner and flatter.
- Near objects: ciliary muscles tighten, and the lens gets thicker and rounder.
This is called accommodation. As people age (usually after 40), the lens stiffens, making close-up reading harder.
The retina: rods and cones
The retina has two kinds of light-sensing cells.
| Rods | Cones | |
|---|---|---|
| Number per eye | About 120 million | About 6 million |
| Work best in | Dim light | Bright light |
| See colour? | No (black, white, grey) | Yes |
| Detail | Low | High |
| Where | Mostly around the edges | Packed in the centre (the fovea) |
There are three types of cones, sensitive to red, green or blue light. Your brain combines their signals to see millions of colours. That’s why screens use red, green and blue pixels.
At night, you mostly use rods. That’s why colours fade in dim light, and why you can sometimes see a faint star better by looking slightly to one side of it.
The blind spot
Where the optic nerve leaves the eye, there are no rods or cones. This creates a blind spot in each eye. You don’t notice it because your brain fills in the gap, and each eye covers the other’s blind spot.
Two eyes: depth perception
Your eyes are about 6 cm apart, so each sees a slightly different view. Your brain combines them to judge distance. This is binocular vision. Try touching two pencil tips together with one eye closed: it’s surprisingly hard!
Vision problems
| Problem | Cause | Correction |
|---|---|---|
| Nearsightedness (myopia) | Eyeball too long; image focuses in front of the retina | Concave lenses |
| Farsightedness (hyperopia) | Eyeball too short; image focuses behind the retina | Convex lenses |
| Astigmatism | Unevenly curved cornea | Specially shaped lenses |
| Colour vision deficiency | One type of cone missing or weak | Special apps and lenses can help |
| Cataracts | The lens becomes cloudy, often with age | Surgery to replace the lens |
Red–green colour vision deficiency affects about 1 in 12 males and 1 in 200 females.
Animal eyes
- Eagles see detail about four to five times farther than people.
- Cats have a reflective layer behind the retina that helps them see in dim light and makes their eyes shine at night.
- Insects have compound eyes with thousands of tiny lenses.
- Mantis shrimp have up to 16 types of colour receptors, compared with our three.
Protecting your eyes
- Wear sunglasses that block UV.
- Never look directly at the Sun.
- Wear safety glasses for sports, labs and power tools.
- Take breaks from screens (try the 20-20-20 rule: every 20 minutes, look at something 20 feet, about 6 m, away for 20 seconds).
- Get regular eye exams.
Common mistakes
- Thinking the lens does all the focusing. The cornea does most of it.
- Thinking we see with our eyes alone. The brain builds the image.
- Thinking the image on the retina is right-side up.
Try it yourself
- Find your blind spot: draw an X and a dot 8 cm apart. Close your left eye, stare at the X with your right eye, and move the paper slowly toward you until the dot disappears.
- Look in a mirror in a dim room, then turn on a bright light. Watch your pupils.
- Hold a pencil at arm’s length and line it up with a distant object using both eyes. Close one eye at a time. Which is your dominant eye?
Quick recap
- Light enters through the cornea and pupil and is focused by the cornea and lens.
- The retina turns light into signals using rods (dim light) and cones (colour).
- The optic nerve carries signals to the brain, which interprets the image.
- Two eyes give depth perception.
- Glasses, contacts and surgery correct common vision problems.