Purpose:Demonstrate constructive and destructive interference using pulses.
Starting identical pulses from both ends simultaneously, either in or out of phase, they can be observed as they pass. For two identical pulses, move your hand rapidly down and up at the center of the machine with the two ends fixed. The two pulses created will reflect off the ends (left photograph) and interfere constructively as they cross each other on their return (right). Repeat this with one end clamped to get a phase reversal of the pulse which reflects off that end.
• This is a good opportunity to bring up one or two volunteers from the class to participate, rather than trying to reach both ends simultaneously yourself.
• Encourage the class to predict what will happen when the pulses pass each other.
In a linear medium like this, two waves moving in opposite directions can be seen to pass through each other. The principle of superposition states that when two mechanical waves pass each other in a medium, the net displacement at any point is the sum of the individual wave displacements.