What it shows
A pencil standing in a glass of water looks broken at the surface because light bends when it passes from water into air. Different liquids bend light by different amounts, and a coin hidden under the rim of a cup reappears when water is poured in.
The science
Light travels more slowly in water or glass than in air. When a light ray crosses the surface at a slant, one side of it slows down before the other, so the ray changes direction; this is refraction. Light from the part of the pencil under water bends as it leaves the water, so your eye sees that part in the wrong place and the pencil looks broken. Thicker liquids such as oil slow light more and bend it more. The coin trick works because light from the coin bends at the surface and reaches your eye over the rim.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A straight pencil or a straw
- Two clear glasses
- Water and some cooking oil
- A coin and an opaque cup
- A notebook for drawings
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
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.
Fill one glass two-thirds with water and stand the pencil in it, leaning against the side.
Look from the side, level with the surface, and sketch how the pencil appears to break at the waterline.
Look from above and notice the underwater part looks shorter and shifted.
Pour cooking oil into the second glass, stand the pencil in it and compare the amount of bend with water.
Carefully pour a layer of oil on top of the water in the first glass and see the pencil bend twice, once at each surface.
Place the coin in the bottom of the empty opaque cup and step back until the rim just hides it from view.
Without moving, have a friend slowly pour water into the cup and watch the coin come back into view.
Draw a diagram with a ray of light from the coin bending at the water surface and reaching your eye.
What you should see
The pencil appears sharply bent at the surface of each liquid, more in oil than in water, and the hidden coin becomes visible when the cup is filled with water.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the pencil look broken?
Light from the underwater part bends as it leaves the water, so your eye sees that part in a different place.
2What is this bending called?
Refraction.
3Why did the coin reappear?
Light from the coin bends at the water surface and now reaches your eye over the rim.
4Why does a pool look shallower than it is?
For the same reason: refraction makes the bottom appear higher than it really is.
Take it further
Shine a torch beam through a glass of water onto a wall and trace how the beam shifts when the water is added.
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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