Light enters the eye through the cornea and the pupil, the lens focuses it as an upside down picture on the retina, and the optic nerve tells the brain. How we see is hard to explain with a still diagram. Put this on the board and the class can follow the light from a candle into the eye: bent at the cornea, through the pupil, focused by the lens and landing upside down on the retina. Then move the candle near and far and watch the lens change shape, or dim the room and watch the pupil open up.
Step through it: one sentence per step, with that part of the eye lit up. Near and far: move the candle and watch the ciliary muscle, the fibres and the lens change, while the picture on the retina goes soft and sharpens again. Bright and dim: the iris muscles tighten and the pupil gets smaller or bigger. Predict, then reveal: on step 5 the class votes on which way up the picture will land before you press Show me. Press a number on the model to jump to that step, hide the labels to quiz the class, and turn the model by dragging. Share copies a link that opens it at the step you are on. More detail, top right, adds the GCSE names for ages 11 and up: the sclera, choroid, aqueous and vitreous humour, suspensory ligaments and blind spot, each with a short explainer when pressed.
No. Nothing comes out of the eye. Light from a source, such as the candle's flame, travels in straight lines into the eye, or bounces off an object first. Plenty of children think it works the other way round, with something travelling out from the eye to the thing they are looking at. Step 1 is there to settle it: the light starts at the candle, and the lines only ever travel towards the eye.
Rods and cones are the two kinds of light sensor in the retina. Rods work in dim light and see in shades of grey. Cones see colour and detail but need brighter light. The fovea, a small dip opposite the pupil, is packed with cones, so it is the sharpest spot. Where the optic nerve leaves there are no rods or cones at all: that is the blind spot. Press Retina, Fovea or Optic nerve on the model to read about each.
The words are pitched at ages 9 to 14: Years 5 to 9 in England, 4th to 8th grade in the United States. In England it fits Year 3 and Year 6 light (“we see things because light travels from light sources to our eyes or from light sources to objects and then to our eyes”), Key Stage 3 physics (the human eye) and AQA GCSE biology 4.5.2.3, where the lens changes shape for near and far. In the United States it supports NGSS 1-PS4-2, 4-PS4-2, 4-LS1-2 and MS-LS1-8.
Why is the image upside down on the retina?
Light travels in straight lines. Light from the top of the candle goes down through the pupil and lands low on the retina, and light from the bottom goes up and lands high. The lines cross inside the eye, so the picture is upside down. Your brain makes sense of it, so you see the world the right way up.
Why do pupils get smaller in bright light?
The pupil is a hole in the iris. In bright light a ring of muscle in the iris tightens and makes the hole smaller, so less light gets in. In dim light other iris muscles tighten and make the hole bigger, so more light gets in. It is a reflex: you do not have to think about it.
How does the eye focus on near and far things?
The lens changes shape. For something near, the ring of ciliary muscle tightens and gets narrower, the fibres holding the lens go slack and the lens gets fatter. For something far away, the muscle relaxes, the fibres pull tight and the lens is pulled thin. At GCSE this is called accommodation, and the fibres that hold the lens are the suspensory ligaments.
Does the cornea or the lens do most of the focusing?
The cornea, the clear window at the front, bends the light most. The lens does the fine focusing, changing shape for near and far things.
What does the simulation show?
A 3D eye cut in half and seen from the side, with a candle in front of it. Bright lines show light from the tip of the flame and the bottom of the candle bending at the cornea and the lens, crossing, and landing as a small upside down picture on the retina. Then a message runs along the optic nerve to the brain.
What has been simplified?
The light is slowed right down so the class can follow it. The candle is much closer than a real far away thing, and the lens changes shape more than a real one so the change shows. Light is shown from just two points of the candle, where every point really sends out light. The real eye is filled with clear jelly, and the picture on the retina is also swapped left to right, which a side view cannot show.
What age is it for?
The words are pitched at ages 9 to 14. It also works as a starter for the eye at GCSE.
Is it free?
Free to try. Opening Science Simulations uses one token for the whole day, however many times you come back to it. Every account gets 20 free tokens, and Pro gives 50 a day.
The pupil shrinking in bright light is a reflex, and the reflex arc simulation shows how one works. The part of the brain that makes sense of what we see is at the back, on the interactive brain model. All twelve are on the Science Simulations page. For models the class can spin, from skeletons to dinosaurs, try 3D Explorer.