Ripple tank and accessories
Teaching Guidance for 14-16
The usual accessories will be required with the ripple tanks:
- light source
- supports for the light sources
- low voltage power supply
- deep beaker
- 1000 cm3 beaker of water
Health & Safety
Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.
Place the power supply for the lamp on a bench, not on the floor by the tank.
Lascells now does a compact and easy-to-use Ripple-Strobe Tank. No trays, legs, motors, lamps or power packs. Self-contained with built-in, synchronized strobe illumination. Variable frequency and wavelength. Just add water, plug in and go.
Setting up a ripple tank
The tank: Wooden tanks, which have not been used for a time, sometimes leak just like a boat which has been taken out of the water. Either fill these up ahead of the lesson or clear up the mess with a sponge. Plastic tanks easily scratch. Some tanks are designed to sit on an overhead projector, for demonstration experiments.
The lamp: Make sure you have the right one. The lamp for a ray box has a vertical filament so that when it is used with cylindrical lenses then the filament is parallel to the vertical axis of the lens. The lamp for a ripple tank has a horizontal filament so that when it is in use the filament is parallel to parallel ripples. If it is possible to darken the room, the ripples will be particularly effective and clear to see. If not, it is desirable to use 48-watt lamps.
Beaches: You may want to use beaches at the edge of the tank to cut down unwanted reflections. These can be made of metal strips with holes in them, gauze or even those green pan scrubs, cut into narrow strips.
Power supplies: Some manufacturers supply special power units for use with their ripple tanks. These provide the necessary voltage for the lamps and also a variable voltage output to drive the motors. Some teachers will use these, while others will prefer to use a transformer for the lamp, with a dry cell and rheostat for the motor.
Setting up: Water level: To avoid unwanted vibrations, ripple tanks should be set up on the floor. Each tank should be filled to a depth of about 5-6 mm using about 1000 cm3 of water. Add a drop (literally) of detergent to the water; this helps to "wet" the wave bar.
Levelling: Varying water depth leads to refraction if the wave bar is not perpendicular to the long axis of the tank. Even with good alignment, when the depth changes this leads to a change of wave speed and hence wavelength. The tanks can be levelled quickly and easily by matching the two reflections of the lamp, one from the glass and one from the water surface. Alternatively, touch the surface with a pencil and check that it produces a perfectly circular pulse.
Position of the wave bar: Adjust the position of the wave bar and/or its frequency so you get standing waves between it and the rear edge of the tank. The resulting increased amplitude of the wave bar helps to get better amplitude waves where you want them.
The lamp: To see the ripples, place it:
- above the tank to cast a shadow onto a large piece of white paper (or a piece of hardboard painted white) underneath the tank.
- or, below the tank to cast a shadow onto the ceiling; this will produce a crick in the neck but is better for class demonstrations.
Adjust the lamp height to give the best picture. It should be about 50 cm above the tank.
Viewing: It is not wise to view the paper through the water; the pattern on the water and the pattern on the paper may combine to produce Moiré fringe effects.
Emptying the tank: A quick way of emptying the tank is to put a rubber tube on the outlet and tilt the tank so that the end of the tube is under water. The tube may be left on as a permanent fixture, if a clip is fastened to it.