How your eye sees: from cornea to retina, step by step

How to use this simulation

1. Press play and drag the object distance slider. As the object comes closer, the ciliary muscle turns red and thickens, the suspensory ligaments go slack, the lens bulges and its power P rises. The image stays on the retina the whole time.
2. Drag the light intensity slider. More light and the iris squeezes the pupil down; less light and it opens wide. The same thing happens in your own eye when you switch a room light on or off.
3. Choose myopia or hypermetropia under the type of eye. The image now lands in front of or behind the retina and blurs. Tick “Wear the correcting lens”: a concave or convex lens moves the image back onto the retina.
4. Click any part of the drawing, or choose it from the list. It lights up and its job appears below. On a small screen the names become numbers, with the key under the picture.
5. Watch the readings for lens power, focal length, near point and far point. Slide the object to 10 cm and even the normal eye blurs: that is inside the near point.

Try this first: make a dot vanish

Draw a small plus sign on a sheet of paper and a thick dot 8 cm to its right. Hold the paper at arm’s length, close your left eye and keep your right eye fixed on the plus. Now bring the paper slowly towards your face without looking at the dot. At about 30 cm the dot suddenly disappears. Move closer still and it comes back.

The arithmetic is simple. The blind spot sits about 15° to the side of the fovea, so the dot lands on it at a distance of 8 ÷ tan 15° ≈ 30 cm. You just found a place in your own eye where there are no light sensors at all.

You never notice it in daily life because the two eyes have their blind spots in different places and the brain fills the gap from the surroundings. This page takes the eye apart. Another oddity is coming too: the picture on your retina is completely upside down, yet you see the world the right way up.

The eye is a living camera

Comparing the eye with a camera makes the parts easier to remember. Both let light into a dark box and form a picture on a screen. The difference is that the eye adjusts its own light, its own focus and its own screen.

Meet the parts: one job each

The human eyeball is nearly spherical, about 2.3 cm across. Let us follow the light from the outside in. Read the six groups below, then find all twelve parts in one table. You can also click each part in the simulation to check.

How the image forms

Say there is a tree in front of you. Light spreads from every point of the tree, and a part of it enters your eye. Here is the journey, step by step.

• Light strikes the cornea. The cornea bends it the most, and the rays start to come together.
• It passes the aqueous humour and reaches the pupil. The iris sets how much light gets through.
• The lens bends the light further and brings it to a fine focus. This is where the focus changes for near or far objects.
• Through the vitreous humour the light reaches the retina and forms a small, inverted, real image.
• Rod and cone cells turn the light into electrical signals, which the optic nerve carries to the brain.
• The brain reads the upside-down picture the right way up, and compares the two eyes to judge depth.

Accommodation: changing focus from near to far

The image distance v is fixed: 2.5 cm from lens to retina. Yet the object may be 25 cm away one moment and a hundred metres the next. The lens formula says that if v stays put and u changes, f must change too. The eye does exactly that by changing the curvature of its lens. This ability is called accommodation.

For a distant object 1/u is almost zero, so 1/f = 1/2.5 = 0.40 cm⁻¹. That gives f = 2.5 cm and a power P = 40 D. For an object 25 cm away, 1/f = 0.40 + 0.04 = 0.44 cm⁻¹, so f = 2.27 cm and P = 44 D. Over the whole range the ciliary muscles change the lens power by only 4 D. Because the cornea stays fixed, that small change is enough.

The nearest distance at which the eye still sees clearly is the near point, 25 cm for a normal eye (also called the least distance of distinct vision). The farthest point is the far point, which is infinity for a normal eye. So the normal eye sees clearly from 25 cm to infinity.

The pupil reflex: sunglasses that run themselves

Walk into a dark room and at first you see nothing, then things slowly appear. Step back into the light and you are dazzled before it settles. The pupil is changing size with the light. In bright light the circular muscles contract and the pupil narrows, down to 2 mm in this simulation. In the dark the radial muscles pull the pupil wide open, up to 8 mm.

Going from 8 mm to 2 mm cuts the area of the opening by a factor of 16, because area grows with the square of the diameter. At 50% light the simulated pupil is 5.0 mm. The brain runs this reflex without thinking about it; you cannot widen or narrow your pupils on purpose.

Adapting to the dark takes much longer, around half an hour, because the pigment in the rods has to be rebuilt. The pupil answers in seconds; the pigment is the slow part.

Rods and cones: two kinds of workers in the retina

The retina has two main kinds of light-sensitive cell. Rods respond to very dim light but cannot tell colours, which is why everything looks grey in faint light at night. Cones need bright light and tell colours and fine detail. There are three kinds of cone, most sensitive to red, green and blue light, and the brain builds every colour from their mix.

Eye defects and how lenses fix them

When the image does not land exactly on the retina, vision blurs. The simulation models the two main defects in the simplest way: the focusing system is the same, only the distance to the retina differs. A correcting lens is an extra lens that changes the direction of light a little before it enters the eye.

Try it in the simulation

Each experiment takes under a minute, and each one makes an idea stick.

Solved problems

Every answer is computed from the simulation’s own model. In the cartesian sign convention the object is to the left of the lens, so u is negative; the image is on the retina, so v = +2.5 cm.

Common mistakes

Most lost marks on this topic come from the same few slips. Check your answers against them before you hand in.

• Drawing the retinal image upright. It is inverted, real and diminished; the brain turns it round.
• Saying the lens moves in accommodation. It does not move; its curvature changes.
• Saying the ciliary muscle relaxes for near vision. For near vision it contracts and the lens thickens.
• Writing a convex lens for myopia. Myopia needs a concave lens; hypermetropia needs a convex one.
• Thinking myopia means “cannot see near”. It is the reverse: near is clear, far is blurred.
• Giving the lens most of the refraction. Most of it happens at the cornea; the lens only fine-tunes.
• Mixing up the fovea and blind spot. The fovea has the most cones and the sharpest vision. The blind spot has no sensors at all.
• Swapping rods and cones. Rods handle dim light with no colour; cones handle bright light and colour.
• Getting the unit wrong. One dioptre is 1 ÷ focal length in metres. With f in centimetres use 100 ÷ f.
• Giving a concave lens a positive power. A concave lens has a negative f and a negative P.

Looking after your eyes

These are textbook-level tips, not medical advice. If you think something is wrong with your eyes, see an eye doctor.

Long close work tires the ciliary muscles. Every 20 minutes, look at something far away for 20 seconds; the muscle relaxes and rests. Read in good light with the book about 25 cm from your eyes. Reading lying down or in a moving vehicle keeps changing the distance and strains the eyes.

Never look straight at the Sun or at very bright light; it can burn the retina. Do not rub your eyes with dirty hands, because the cornea scratches easily. If something gets into your eye, rinse it with clean water.

Eat foods rich in vitamin A, such as carrots, sweet pumpkin, eggs, liver and leafy greens. A check-up once a year catches short or long sight early.

Donated eyes can give sight to another person through a cornea transplant. Only the cornea is transplanted, and it must be done within a few hours of death. It is worth talking to your family about it beforehand.

Exam corner: board exams

The eye is a topic where biology and physics meet, so questions can come from both. Biology asks for structure and function; physics asks about refraction, lens power and the defects of vision.

Recall questions: “Define accommodation”, “What is the blind spot?”, “What is the unit of lens power?”. Diagrams: a vertical section of the eye with the cornea, iris, pupil, lens, ciliary muscle, vitreous humour, retina, fovea and optic nerve labelled. Name the parts in the order light meets them and the labels fall into place.

Reasoning questions: “Why does the ciliary muscle contract for near vision?”, “Why does the pupil widen in the dark?”, “Why do we see upright when the retinal image is inverted?”. Numerical questions: glasses power from the far point or near point. Write 1/f = 1/v − 1/u first, put in the signs, and finish with the unit D. Higher-order questions: explain why a given person needs a concave or convex lens.

Multiple-choice items come back again and again: where most refraction happens, where the cones are concentrated, which lens cures which defect. In comparison tables keep a row each for myopia and hypermetropia: cause, where the image falls, which lens. Always add a diagram and a unit.

Quick revision

Read this the night before the exam; every line is a likely question.

• Path of light: cornea, aqueous humour, pupil, lens, vitreous humour, retina.
• The retinal image is inverted, real and diminished; the brain interprets it upright.
• Most refraction is at the cornea; the lens fine-tunes the focus. The image distance is fixed near 2.5 cm.
• Accommodation: for near objects the ciliary muscle contracts, ligaments slacken, the lens thickens and P is higher (44 D); for far ones it is the reverse (40 D).
• A normal eye has a near point of 25 cm and a far point at infinity.
• The iris changes the pupil: small in bright light, large in the dark.
• Rods: dim light, no colour. Cones: bright light, colour. The fovea has the most cones.
• The blind spot has no sensors, because the optic nerve leaves there.
• Myopia: image in front of the retina, concave lens, negative power.
• Hypermetropia: image behind the retina, convex lens, positive power.

Frequently asked questions