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### Course: Class 11 Physics (India)>Unit 19

Lesson 3: Wave interference

# Wave interference review

Review the key terms and skills related to wave interference, including how to predict wave interference patterns.

## Key terms

TermMeaning
PulseA single disturbance that moves through a medium.
InterferenceWhat occurs when two or more waves pass through one another. Also called superposition.
Constructive interferenceWhen overlapping waves produce a wave with an amplitude that is the sum of the individual waves.
Destructive interferenceWhen overlapping waves produce a wave with an amplitude that is less than the sum of the individual waves.

## How to predict interference for two pulses

When two pulses overlap, we need to add each individual pulse’s displacement to determine the shape of the resulting pulse.

### Constructive interference

We get constructive interference when pulses have the same direction of displacement from equilibrium at the same position. If two pulses are completely overlapping, the resulting pulse has an amplitude that is the sum of the amplitudes of each individual pulse.

### Destructive interference

We get destructive interference when pulses have displacements from equilibrium that are in opposite directions. If the two pulses are completely overlapping, the resulting pulse has a smaller amplitude than either individual pulse. If the pulses have the same amplitude but opposite displacement, then the resulting pulse has zero amplitude.

## Common mistakes and misconceptions

1. Sometimes people forget that amplitudes add up at every point. When pulses meet, the different parts of the pulses can have constructively or destructively interference depending on their relative displacements from equilibrium.
2. People sometimes forget that constructive pulses don't reflect off each other. If we look at wave pulses with displacement in opposite directions, we can tell the pulses continue moving in the same direction after they overlap. The same phenomenon occurs for constructive interference.

For deeper explanations of superposition, see our video on wave interference.
To check your understanding and work toward mastering these concepts, check out the exercise on predicting wave interference patterns.

## Want to join the conversation?

• is pulse and superposition same?
• No, a pulse is a single disturbance in a wave. Superposition, however, is when two pulses cancel out each other. A pulse's amplitude may be 50m at one point in time, but if there is a pulse which has an amplitude of -50m at the same time, they will cancel out and nothing will be heard, that is how noise-cancelling headphones work.
• how does the superposition principle differ from quantum superposition? are there any similarities or conceptual overlap?
• The wording of "superposition" is indeed taken from the wave's superposition property, and I believe they more or less have some similarity, but the main thing they are describing is different.

"...However, unlike classical waves, quantum state amplitudes do not correspond to motion: adding two identical states is not meaningful..." [1]

For wave superposition, it states that when two waves interfere with each other, the result will be the vector summation of the two waves. This explains why destructive interference and constructive interference occurs.

For quantum superposition, it states that an atom can be at excited state and not excited state at the same time. And when you observe it, you can observe 1 state. Which means at one moment you observe the atom, it is at excited state, and at another moment, it isn't.
So quantum superposition simply means that two state can exist in an atom at the same time.

[1] https://en.wikipedia.org/wiki/Quantum_superposition
(1 vote)
• What would happen if a wave "interfered" with another wave at the end of a sting? This string would be connected to a wall with 2 screws, so that when a wave hits the screw, it bounces back, i.e. the pulse(s) never stop. So, if there are 2 pulses that "interfere" with one another at a point where half of each of the pulses is going to the right, and half is being reflected back left, what would the resulting wave be?
• I assuming you mean that when a wave bounces from the wall and when it is exactly half way through its bounce what will happen.

Since when a wave hits a wall, the wave will go upside down. If it is initially above the string, then after the bounce it will be below the string, and vice versa.

By this information, you know that destructive interference occurs in your problem. Thus at that instant, you will observe a totally straight line.
(1 vote)
• can i have tip
(1 vote)