What it shows
The pupil gets smaller in bright light and larger in dim light, an automatic reflex that protects the eye and helps it see. A mirror and a ruler let you measure this change without any special equipment.
The science
The iris is a ring of muscle around the pupil. In bright light the circular muscle tightens and makes the pupil smaller, which reduces the light entering. In dim light the radial muscles pull and widen the pupil. The change is a reflex controlled by the nervous system without any thought.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A small mirror
- A ruler marked in millimetres
- A room where lights can be dimmed
- A phone camera
- A friend
- A notebook
- A soft torch (not a laser)
Safety
For this project: Never shine a strong or direct light into anyone’s eyes. Use only ordinary room light, and an adult must be present. This is not an eye test.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Sit in a dim room for two minutes and let your eyes adjust.
Hold the mirror and the ruler beside your eye and read the width of the pupil in millimetres.
Take a close-up phone photo with a ruler next to the eye for a second measure.
Switch on the room light and wait ten seconds.
Look in the mirror again and measure the pupil width straight away.
Repeat with a friend as a second subject and with five other volunteers.
Calculate the average pupil width in dim and bright light.
Draw a sketch of the eye in both conditions and label the iris and the pupil.
What you should see
Pupils measure a few millimetres wider in dim light, about 5 to 7 mm, than in bright light, about 2 to 4 mm. The class average shows the change is the same for everyone.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the pupil?
The opening in the middle of the iris that lets light in.
2Why does it get small in bright light?
It protects the retina from too much light.
3What kind of action is this?
A reflex action.
4Why not use a torch directly?
Bright light may hurt the eye.
Take it further
Time how long the pupil takes to adjust after a sudden change in room lighting.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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