The ripple tank is a classic piece of physics equipment which is still used in many schools. It consists of a (usually) plastic tray with a transparent bottom which is usually lit from above using a desk lamp. Some small types can be placed over an overhead projector so that the waves can be projected onto a screen for all to see.
Ripple tanks can be used to show all the basis properties of waves including diffraction, reflection, refraction and interference.
Ripples are usually created by a small electric motor attached to a strip of wood suspended by springs or elastic bands. The motor has an off-centre weight attached so when turned on, makes the strip vibrate and thus create ripples on the surface of the water.
By lighting the waveforms being created, clear peaks and troughs can be seen. Trial and error is often needed to get a good image of the waves. Make a note exactly how you get the best results so it can be re-created the next time.
To begin you need to be able to make consistent plane waves. Lower the motor assembly until it just touches the surface of the water and turn on the motor. You should be able to see straight waves being created which travel across the surface of the tank. These are often known as plane waves.
Waves can be reflected using pieces of straight or bent metal strip. Place a straight strip in the water and send a few waves towards it. You should clearly see the waves reflect off the strip obeying the 'law of reflection'
If you insert a concave strip and send a few waves towards it, the waves that are reflected should converge at a specific focal point in the same way as light behaves in a solar furnace.
Take a relatively thick piece of glass or transparent plastic and place it in the path of the plane waves. The glass should still be covered with water enabling waves to travel over the top if it. Because the speed of a wave depends upon the depth of the water, this shallow area slows down the wave. If the piece of glass is placed at an angle to the oncoming wave, refraction will take place. You should be able to see a bend in the plane wave just where the glass is placed.
When waves on the sea travel into a harbour with a narrow entrance, they diffract, making a semi-circular pattern when passing through. This can be created using two pieces of metal strip, leaving a gap between the two. The narrower the gap, the more semi-circular the wave will be when it passes through.
By adding two small balls to the motor assembly, two waves can be created from the point these balls make contact with the water surface. These pretty much identical wave patterns will interfere with each other creating visible light and dark areas where they merge. You should also be ablt to see ‘flat’ areas of destructive interference in places where the waves cancel each other out.
To cut down on unwanted reflections created by the sides of the tank, some technicians use small pieces of sponge, glued in place round the edge to absorb waves so they do not reflect back and interfere with others.
If lit from above, a large piece of white card should be used underneath to clearly show up the wave patterns. If lit from below, a low voltage lamp can be placed under the ripple tank to projects waveforms onto the ceiling which creates a larger viewing area.
Older ripple tanks are prone to leakages where the plastic start to deteriorate. Epoxy resin can be used to glue these small gaps shut.
There are many virtual ripple tanks available either online or contained within some science software. These obviously have many advantages over the conventional ripple tanks for the technician (spilled water, safety and lugging it around) and are great for revision purposes but if set up properly, the original ripple tanks are very effective at showing pupils these basic wave principles in a very hands-on fashion.
Strobe wheels are often included with ripple tanks. They consist of a spinnable wheel with slits cut from the edge. When rotated at speed and looked through with one eye, the image being looked appears to flicker. Waves appear easier to see using this method and standing waves can often be observed.