Inside the eye: from light to a picture
How the cornea, pupil, iris and lens focus light on the retina, rods and cones, three kinds of cone for colour, the blind spot, glasses and colour blindness. With a blind-spot fair test.
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Light first passes through the cornea, a clear dome that bends light. It then goes through the pupil; the iris controls how much light the pupil lets in. The lens works with the cornea to focus the light on the retina.[1]
In bright light the iris contracts and the pupil gets smaller; in dim light the pupil gets bigger. Small muscles also change the shape of the lens so it can focus on near or far things.[2]
The retina has two kinds of light-sensitive cells. Rods are very sensitive and work in dim light, but see only shades of grey. Cones need brighter light and give colour vision. Cones are packed into the fovea, the spot where you see most sharply.[2]
There are three kinds of cone, most sensitive to blue, green and red light. Your brain compares their signals to work out each colour.[3][2]
Where the optic nerve leaves the eye there are no light-sensitive cells: this is the blind spot. You don't notice the gap because your brain fills it in.[3][8]
The picture on the retina is actually upside down; the brain makes sense of it the right way up.[4]
If the eyeball is a bit too long, far-away things look blurry (nearsightedness); if it is too short, near things look blurry (farsightedness). Red-green colour blindness, the most common kind, is inherited and more common in men, because the gene is on the X chromosome.[6][7]
Try it: Find your blind spot
You need
- A white card with a black X on the left and a black O on the right, about 7 cm apart
- A ruler or tape measure
- A partner
- Results table
Safety
Measure to the cheekbone, never point a ruler or pencil at an eye. Keep the card a comfortable distance away and stop if your eyes feel tired.
What to do
- Question: is the blind spot the same distance away for your left eye and your right eye? Write a prediction.
- Hold the card at arm's length, level with your eyes. Close your left eye and stare at the X with your right eye.
- Slowly bring the card closer, still staring at the X. Stop as soon as the O disappears.
- Your partner measures the distance from the card to your cheekbone (not the eye!). Repeat 3 times and find the mean.
- Now close your right eye and stare at the O with your left eye: watch the X disappear. Measure 3 times again.
- Fair test: same card, same light, same partner measuring, staring at the letter all the time. Only the eye changes.
What happens, and why?
At one distance the image of the letter falls exactly on the spot where the optic nerve leaves the retina. There are no light-sensitive cells there, so that part of the picture is missing. Your brain fills in the gap with the white of the card, so the letter seems to vanish.[8][3]
Play: Which part of the eye?
Read each job. Which part of the eye does it?
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Quiz
Other ages: Ages 6–7 · Ages 15–16