How does the eye see?

Your eye is a ball about 24 mm across, a bit smaller than a table-tennis ball. Light comes in through the clear dome at the front, the cornea, crosses a watery fluid, the aqueous humour, and passes through a hole, the pupil. The coloured ring around the pupil is the iris, a muscle that makes the hole bigger in the dark and smaller in bright light.

Just behind the pupil sits the lens. It hangs from hundreds of fine threads, the zonules, which are tied to a ring of muscle, the ciliary muscle. Behind the lens, the eye is filled with a clear jelly, the vitreous humour.

The wall has three layers. The tough white sclera is the outside, the dark, blood-rich choroid feeds the layer inside it, and the innermost layer is the retina, a thin sheet of light-sensing cells. Right in the middle of the back is the macula, with a tiny pit, the fovea, where you see the sharpest detail. A little towards your nose is the optic disc, where about 1.2 million nerve fibres leave the eye as the optic nerve. There are no light-sensing cells on the disc, so it is your blind spot.

Outside, six muscles turn the eye: four straight ones (the rectus muscles: up, down, in and out) and two slanted ones (the obliques). The eyelids sweep tears over the cornea about 15 to 20 times a minute to keep it clean and wet.

If this sounds like a camera, it is: the iris is the aperture, the lens focuses and the retina is the sensor. CameraClear shows the camera side of the story.

Light only makes a sharp picture if every ray from one point of an object lands on one point of the retina. Bending light to do that is called focusing, and it happens at curved, clear surfaces: light bends as it slows down entering the eye (the physics is Snell's law of refraction, which our upcoming SnellClear box explains).

The eye has about 60 dioptres of focusing power. The cornea does about two-thirds of it, around 42 D, because the jump from air into the cornea is the biggest change of speed. The lens adds about 20 D. A dioptre is 1 divided by the focal length in metres.

The cornea is fixed, so the lens does the adjusting. Looking far away, the ciliary muscle relaxes, its ring is wide, the zonules pull tight and the lens is flat. To look at something near, the muscle contracts, the zonules go slack and the springy lens rounds up, adding power. This is accommodation. A camera moves its lens instead: compare CameraClear's focus chapter.

The picture on the retina is upside down and flipped left to right, just like in a camera. Your brain learns to read it the right way up.

With age, the lens stiffens. A 10-year-old can focus on something about 7 cm away; by about 45 the near point has moved past arm's length, which is why many people need reading glasses in their 40s. This is presbyopia, and it happens to everyone.

For sharp distance vision, the eye's length has to match its focusing power almost exactly. The margin is tiny: an eyeball just 1 mm too long is about 3 dioptres short-sighted.

In short sight (myopia) the eyeball has usually grown too long, so light from far away meets in front of the retina and has spread out again by the time it lands. Near things still look sharp. A concave (minus) lens, thinner in the middle, spreads the light out a little first, so it meets further back, on the retina.

In long sight (hyperopia) the eye is too short, so the light would meet behind the retina. A convex (plus) lens, thicker in the middle, adds the missing power. Young people can often squeeze their own lens to make up for it, but that can tire the eyes, especially when reading.

In astigmatism the cornea is shaped more like the back of a spoon than a ball, curved more one way than the other. Lines in one direction come into focus at a different depth from lines in the other. Glasses fix it with a cylinder, a lens that bends light more in one direction.

Short sight is becoming much more common. About 1 in 4 people in the world were short-sighted in 2000, and researchers project about half by 2050. In Indian cities, the share of school children with myopia rose from about 4% in 1999 to about 21% in 2019. Studies link more time outdoors in daylight to less myopia in children. Only an eye test can tell what you need, so see an optometrist or eye doctor if things look blurry.

The retina is thinner than a credit card, but it holds about 120 million rods and 6 million cones (a careful 1990 count found about 92 million and 4.6 million, and people vary). Rods are extremely sensitive and work in dim light, but they can't tell colours apart. Cones need brighter light, give you colour and fine detail. The fovea is packed with cones and has no rods at all, which is why you look straight at things to read them.

Strangely, the retina is wired back to front: light passes through the nerve cells first and is caught by the rods and cones at the very back, next to a dark pigment layer that soaks up stray light. The signal then travels forward through bipolar cells to ganglion cells, whose long fibres run across the retina to the optic disc and on to the brain's visual cortex (see BrainClear).

How does a rod catch light? It is full of a pigment, rhodopsin. When a photon hits one molecule, a small part of it, retinal, snaps into a straighter shape. That switches on a chain of proteins that closes tiny gates in the cell, and the rod sends a changed signal. One photon is enough. This is phototransduction.

Bright light bleaches rhodopsin, and it takes time to rebuild. That is why a dark cinema looks pitch black at first. Your cones adapt within about 10 minutes, then the rods take over and keep improving for about 30 minutes, making you roughly 10,000 times more sensitive.

Where the optic nerve leaves, there are no rods or cones at all: your blind spot. You never notice it, because your other eye covers it and your brain fills in the gap.

You have three kinds of cone. Each holds a slightly different pigment, so each catches some wavelengths better than others: S cones peak in the violet-blue (about 420 nm), M cones in the green (about 530 nm) and L cones in the yellow-green (about 560 nm). Their curves overlap a lot.

No single cone knows what colour it saw: it just sends "how much". Your brain compares the three, and the pattern is the colour. This is the trichromatic theory, first suggested by Thomas Young in 1802.

Here is the trick: light of one pure wavelength, say 580 nm yellow, and a mix of red and green light can give the three cones exactly the same pattern. Your eyes can't tell them apart. So a phone or TV only needs red, green and blue subpixels to fool you into seeing every colour: see TVClear's pixels.

In colour vision deficiency ("colour blindness"), one cone type is missing or shifted. Most common is red–green, which affects about 8% of men and 0.5% of women of Northern European ancestry, because the genes sit on the X chromosome. Rates differ between populations: studies in India find roughly 4 to 9% of boys and men, depending on the community. Most colour-blind people see many colours, just not all the differences. A simple eye test can check it.

This chapter shares general facts only. If you notice any change in your sight, see an optometrist or eye doctor. Sudden loss of vision, a shower of new floaters or flashes, or a painful red eye need help the same day.

Cataract. With age the lens slowly turns cloudy and yellow, like a frosted window. Things look blurry and dull, and lights glare. Cataract is the leading cause of blindness in the world and in India. The fix is one of the most common operations anywhere: the surgeon removes the cloudy lens and puts in a clear plastic intraocular lens (IOL). India does millions of them every year, many free.

Glaucoma. The eye is always making clear fluid, the aqueous humour, which drains out at the edge of the iris. If it drains too slowly the pressure can rise and slowly damage the optic nerve, so the pale cup in the optic disc grows. Vision fades from the edges first, so most people don't notice until a lot is lost, and lost sight can't come back. Regular eye checks, especially after 40 or if it runs in the family, catch it early.

Diabetic retinopathy. Years of high blood sugar damage the retina's tiny blood vessels. They leak and bleed, and fragile new ones can grow. People with diabetes are advised to have a dilated eye check every year.

Screens and sun. Staring at a phone keeps your ciliary muscle squeezing and you blink less, which can leave eyes tired and dry, though no lasting damage is known. The 20-20-20 rule, recommended by the American Academy of Ophthalmology and widely credited to optometrist Jeffrey Anshel: every 20 minutes, look at something 20 feet (6 m) away for 20 seconds. Time outdoors helps children's eyes, sunglasses that block UV protect the lens and retina, and never look straight at the Sun.