# Simple circular pulses in ripple tanks

##### Demonstration

Good questions will encourage close observations of circular pulses in a ripple tank.

Pencil

Water Dropper

#### Health & Safety and Technical notes

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.

#### Procedure

Start a single ripple somewhere in the middle of the tank and then making several such ripples one after the other. Show this can be done using:

• finger
• pencil to touch the water
• drop of water from an eye dropper

• What is the shape of the pulse as it travels out?
• Is the speed of travel the same in all directions? How can you tell?
• Is the speed the same near to the centre of the tank and at the edges?
• Is the water moving along with the wave pattern? Drop a scrap of paper onto the water to see if it travels.

Let students suggest their own tests. Any materials that students suggest for a test can be fetched quickly; and if they suggest none, the problem had best be left unsettled.

#### Teaching notes

1 Possible learning outcomes from this experiment:

• Students become more familiar with the ripple tank.
• They find that the water does not move out with the ripple.
• They learn what is meant by a circular ripple.
• They deduce that it is circular because – if the water is of constant depth – the disturbance travels with the same speed in all directions.
• They might deduce what happens to the ripple speed if the water depth increases.

2 Pulses will be circular if the tank is level. A circular wave train demonstrates that waves travel the same distance in the same time.

This experiment was safety-checked in February 2006