# Reflection of a circular pulse by a barrier

##### Demonstration

You can use a ripple tank to introduce ideas about the relationship between an ‘object’ and its mirror ‘image’.

#### Apparatus and materials

For each student group

Straight barrier

Water dropper

Pencil

#### 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.

You can use either a water dropper or a pencil (dipped in the surface of the water).

#### Procedure

Ask: ‘When a circular ripple (pulse) is bounced back by a straight wall, where does the wave seem to come from after that?' Do not give the answer to that, and do not, at this stage, discuss images. Instead, ask students to carefully observe this process in their ripple tanks.

Tell them to place the barrier somewhere near the middle of the tank (so that the image from which the reflected ripple seems to come is well inside the tank).

When students have seen this for themselves, suggest some further experiments:

• 'Now that you know where the ripple that bounces back seems to come from, try starting a ripple just there. Use the finger of one hand. Let the ripple spread and hit the wall and bounce back. Mark the place where the bounced back ripple seems to come from, with a finger of your other hand.
• Then start a ripple from that place with that finger.
• 'Now start ripples with both those fingers at the same moment. Watch what happens '

When that succeeds, it is amusing and almost uncanny. Students may notice the geometry, but it will not matter if they fail to notice it. The main aim here is to emphasize the idea of a ' place from which the reflected wave seems to come'.

#### Teaching notes

1 Do not spoil the fun by doing it for students even though you can probably make the simultaneous ripples (the incident ripple and the image ripple) much more easily. Do, however, help students who are unsuccessful by encouraging them to put the second finger at the right place - without using the word image or giving the geometry. Just judge the right distance with your own eyes and point to the right place. Then the student can try again, see success and enjoy it.

2 Students who find it difficult to remember the position of 'the place the reflected ripple comes from' (the image) could put a small coin in the tank at that spot. More able students could measure the distances of the ‘object’ and ‘image’ from the barrier after marking their positions.

This experiment was safety-checked in May 2006