# Interference with two sources, using fingers

##### Class Practical

Using a ripple tank to observe interference between two wave pulses that are in phase may be suitable for intermediate or advanced level students.

#### Apparatus and materials

Light source, compact

Paper, white

#### 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.

The white paper is used as a screen on the floor.

#### Procedure

a Use two fingers of one hand to make a pair of single pulses, at the same time.

b Now make two streams of continuous ripples. Take turns observing with a hand stroboscope the pattern this produces. Can you see any pattern? Can you explain what you see?

#### Teaching notes

1 This method will not produce a pattern that is very regular or easily seen, but it should give a feel for interference from two sources.

The exercise could lead on to the use of the vibrator with two point dippers attached to it to act instead of the fingers.

2 Where the two waves overlap, they simply give the sum of their two separate effects. If those effects are both in the same direction the sum is large, called ‘constructive interference’. If the effects are in opposite directions then the sum may be small or even zero, called ‘destructive interference’.

3 The explanation that you give will depend on student’s prior knowledge. A useful discussion with students might run like this:

here one ripple arrives that makes the water go up and down

flip-flap, flip-flap.....

and the other ripple also arrives making the water go up and down

flip-flap, flip-flap.....

The two wave signals add up, FLIP-FLAP, FLIP-FLAP

But here, where the wave from one source has travelled a little further than the wave from the second source, the two waves do not arrive in step.

One makes the water go up and down flip-flap

and the other makes the water go flap-flip

Or you may prefer to use more conventional terms, identifying crests and troughs - or simply up and down. You could also use plastic wave models to explain what is happening. These are described in Collection 8.

The most important thing is to draw students attention to places where each of these happen.

4 With more advanced students, you may also want to draw attention to distances from the two sources at various locations, and the way that different path lengths affect the relative phases of the waves from each source.

This experiment was safety-checked in February 2006