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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.

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.

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.

Experiment: Find your blind spot

1. Question: is the blind spot the same distance away for your left eye and your right eye? Write a prediction.
2. Hold the card at arm's length, level with your eyes. Close your left eye and stare at the X with your right eye.
3. Slowly bring the card closer, still staring at the X. Stop as soon as the O disappears.
4. Your partner measures the distance from the card to your cheekbone (not the eye!). Repeat 3 times and find the mean.
5. Now close your right eye and stare at the O with your left eye: watch the X disappear. Measure 3 times again.
6. Fair test: same card, same light, same partner measuring, staring at the letter all the time. Only the eye changes.

Eye	Try 1 (cm)	Try 2 (cm)	Try 3 (cm)	Mean (cm)
Right eye (looking at X)
Left eye (looking at O)

Questions

1. Put in order the parts light passes through: lens, cornea, retina, pupil.

2. Give one difference between rods and cones.

3. Write your mean blind-spot distance for each eye. Were they about the same?

4. Why don't you notice your blind spot in everyday life?

Sources for every fact on this sheet: echoesof.science/school/eyes/sources/

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