Water waves seen in section
Practical Activity for 14-16
Another way of demonstrating the transverse motion of particles in a water wave.
Apparatus and Materials
- Tank, large rectanglar transparent
- Paddle (wood block with handle)
- Sawdust, fine, small amount
- Paraffin, coloured
Health & Safety and Technical Notes
Fine sawdust in the air is hazardous. Provide only a small quantity (e.g. 100 ml) in a small beaker or jar.
A long tank is preferable to a short one, so that the initial outgoing waves are not immediately affected by the waves reflected from the end. The ideal is depth 15 cm, breadth 7 cm, length 1 m.
The paraffin layer on top of the water layer has to be added carefully so that air bubbles are not trapped between the two layers. Attach a funnel to some rubber tubing and place the other end of the tubing on the water surface. Pour paraffin gently into the funnel to produce a transparent layer on top of the water.
Cleaning the tank is very messy but a strong detergent will do the job. Decant the paraffin off the top and store it until the next use.
- Fill the glass tank half full with water and place it so that the students can see the water line face on and any waves passing along it in cross-section.
- Generate waves at one end by moving the hand or a block of wood up and down in the water or, better, sweeping it back and forth as a paddle.
- The students watch the motion.
- If you mix some sawdust in the water those watching at close range may see the path of the particles of the medium when the water waves travel along. However, that motion is too fast to see easily so try step 5.
- Fill the tank one-third full of water and add paraffin (preferably coloured) above that until the tank is two-thirds full. Generate transverse waves at the interface, keeping the paddle immersed.
- Before looking down on waves (ripples) in ripple tanks, it is useful to look at water waves in section in a long plastic tank.
- To produce waves at the interface of the water and paraffin, oscillate the paddle in the water only. Try to avoid producing waves at the air/paraffin interface. With slower wave velocities it may be possible, by adding fine sawdust, to show that the motion of the water ‘particles’ close to the surface moving in circular paths and those in deeper water moving in elliptical paths.
This experiment was safety-tested in February 2006