The eye and how it focuses

Light crosses the cornea, the pupil and the lens before it lands on the retina, where two kinds of cells turn it into nerve signals.

The path of light

Look up from this page at something across the room and then back again. For a moment the words blur, and then they are sharp. In that half second a ring of muscle inside your eye changed the shape of its lens. This lesson follows light through the eye and shows how the eye keeps both near and far things in focus.

The National Eye Institute describes the route in a few steps. Light first passes through the cornea, the clear dome at the front, which bends the light to help the eye focus. Some of it then goes through the pupil, the opening in the middle of the coloured iris. The iris is smooth muscle: it makes the pupil smaller in bright light and larger in dim light, so it controls how much light gets in. Next the light passes through the lens, which works with the cornea to bring the light to a point on the retina at the back of the eye.

The retina is where light becomes a nerve signal. Two kinds of photoreceptor cells do this. Rods work in dim light and do not sense colour; OpenStax notes that they are so sensitive that a single photon can make a signal. Cones need brighter light and come in three types. By comparing how strongly each type responds, the brain works out colour. The signals leave the eye through the optic nerve and travel to the brain, which turns them into the images you see.

Three layers and two fluids

The wall of the eyeball has three layers. The outer layer is the tough white sclera, which covers about five sixths of the surface, together with the clear cornea at the front. The middle layer holds the choroid, full of blood vessels, along with the ciliary body and the iris. The inner layer is the retina. The space in front of the lens is filled with a watery fluid, the aqueous humor, and the larger space behind it holds a thicker gel, the vitreous humor. Tears from the lacrimal gland keep the front surface wet and wash away small particles.

Focusing near and far

A camera focuses by moving its lens back and forth. The eye changes the shape of its lens instead. The ciliary body is a ring of muscle joined to the edge of the lens by fine fibres called zonules. When you look at something far away, the lens is pulled flatter. When you look at something close, the lens becomes rounder and bends light more strongly. In the 3D model you can watch the lens thicken as the lamp comes closer.

Sometimes the light does not land exactly on the retina. The National Eye Institute calls these refractive errors. In nearsightedness (myopia) far-away things look blurry because light focuses in front of the retina, because the eyeball has grown too long from front to back or because of the shape of the cornea or lens. It usually starts between ages 6 and 14. In farsightedness (hyperopia) nearby things look blurry because light focuses behind the retina, because the eyeball is too short or because of the shape of the cornea or lens. Presbyopia is different: after about age 45 the lens gets harder and less flexible, so it cannot become round enough for close work. Glasses or contact lenses are the most common correction.

The sharp spot and the blind spot

Keep your eyes on one word in the middle of this paragraph. The words at the start and end of the lines are hard to read without moving your eyes. That is because the centre of the retina has a small pit, the fovea, where cones are packed tightly together with no blood vessels in the way. Your eyes keep moving so that whatever matters lands on the fovea. A little to the side is the optic disc, where the nerve fibres leave the eye. It has no rods or cones at all, so every eye has a blind spot. You can find yours with the card below.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.