What it shows
A shallow tray of water lit from above shows ripples as bright and dark lines on white paper below. Ripples reflect from a barrier and bend when they pass from deep to shallow water, because waves travel more slowly in shallow water.
The science
Ripples are transverse waves on the surface, and the speed depends on the depth: for shallow water the speed is roughly proportional to the square root of the depth. A wavefront entering a shallower region slows down, its wavelength shortens, and its direction changes, which is refraction. A wave hitting a barrier reflects with the angle of incidence equal to the angle of reflection. Water acts as a lens, so the crests focus light into bright lines on the screen.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A clear glass or plastic tray, about 30 cm by 20 cm, or a glass baking dish
- Water
- White paper or a sheet of white card for the screen
- A bright LED torch or phone light
- Four books to raise the tray
- A ruler, a pencil and a dropper
- A flat piece of plastic about 2 mm thick
Safety
For this project: Keep the lamp and phone away from water and wipe spills at once. Use only battery lights, never a mains lamp near the water. An adult must supervise.
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.
Stack the books to make two supports and place the tray across them so that there is a gap below for the paper screen.
Pour water about 1 cm deep into the tray and let it become still.
Hold the torch about 40 cm above the tray, pointing down. Place the white paper below and adjust until you see a clear circular spot of light.
Touch the water surface with a pencil tip once a second at one end to produce circular ripples, and watch the pattern on the paper.
Use a ruler to make straight waves by dipping its edge at one end of the tray. Place a block of clay or a book as a barrier and watch the reflection.
Place the plastic sheet flat in the tray, covering half the area so that the water over it is very shallow, and send straight waves at an angle.
Notice the change in the direction and the spacing of the lines at the edge of the shallow part, and sketch what you see.
Use a ruler on the screen to measure the wavelength in deep and shallow parts and compare.
What you should see
The ripples reflect from the barrier at an equal angle, and in the shallow region the wavelength is smaller and the wavefronts bend toward the normal.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why do waves bend in shallow water?
They travel more slowly there, so the wavefront changes direction.
2What is the law of reflection?
The angle of incidence equals the angle of reflection.
3What happens to the wavelength in the shallow region?
It decreases because the speed is lower and the frequency stays the same.
4How can you tell the frequency is constant?
It is set by the source, so it stays the same on both sides.
Take it further
Use two dippers together to show interference between two sets of circular waves.
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.
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