# Interference

## Interference occurs when multiple waves interact with each other, and is a change in amplitude caused by several waves meeting.

#### Key Points

• Interference is a phenomenon of wave interactions. When two waves meet at a point, they interfere with each other.

• There are two types of interference, constructive and destructive.

• In constructive interference, the amplitudes of the two waves add together resulting in a higher wave at the point they meet.

• In destructive interference, the two waves cancel out resulting in a lower amplitude at the point they meet.

#### Terms

• The maximum absolute value of some quantity that varies.

• A vector quantity that denotes distance with a directional component.

• Of waves having the same direction, wavelength and phase, as light in a laser.

#### Figures

1. ##### Constructive Interference

Pure constructive interference of two identical waves produces one with twice the amplitude, but the same wavelength.

2. ##### Superposition

Pure destructive interference of two identical waves produces zero amplitude, or complete cancellation.

3. ##### Interference

Two overlapping waves exhibit interference.

Unlike solid objects, two waves can share a point in space. In physics, interference is a phenomenon in which two waves (passing through the same point) superimpose to form a resultant wave of greater or lower amplitude. Interference usually refers to the interaction of waves that are correlated or coherent with each other (i.e, "interfere" with each other), either because they come from the same source or because they have the same or nearly the same frequency.

The effects of interference can be observed with all types of waves, for example, light, radio, acoustic and surface water waves (Figure 3). The idea that interference is caused by superposition means that when two waves meet their two amplitudes (their maximum absolute value) combine together.

Interference can be constructive or destructive. In constructive interference, the two amplitudes of the waves add together and result in a higher displacement than would have been the case if there were only one wave. An example of constructive interference may be seen in Figure 1.

Destructive interference is when two waves add together and the result is a smaller displacement than would have been the case. An example of destructive interference can be seen in Figure 2. When the waves have opposite amplitudes at the point they meet they can destructively interfere, resulting in no amplitude at that point. For example, this is how noise cancelling headphones work. By playing a sound with the opposite amplitude as the incoming sound, the two sound waves destructively interfere and this cancel each other out.

#### Key Term Glossary

amplitude
The maximum absolute value of some quantity that varies.
##### Appears in these related concepts:
coherent
Of waves having the same direction, wavelength and phase, as light in a laser.
##### Appears in these related concepts:
constructive interference
Occurs when waves interfere with each other crest to crest and the waves are exactly in phase with each other.
##### Appears in these related concepts:
destructive interference
Occurs when waves interfere with each other crest to trough (peak to valley) and are exactly out of phase with each other.
##### Appears in these related concepts:
displacement
A vector quantity that denotes distance with a directional component.
##### Appears in these related concepts:
Displacement
The length and direction of a straight line between two objects.
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frequency
The quotient of the number of times n a periodic phenomenon occurs over the time t in which it occurs: f = n / t.
##### Appears in these related concepts:
interfere
(of waves) To be correlated with each other when overlapped or superposed.
##### Appears in these related concepts:
interference
An effect caused by the superposition of two systems of waves, such as a distortion on a broadcast signal due to atmospheric or other effects.
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light
The natural medium emanating from the sun and other very hot sources (now recognised as electromagnetic radiation with a wavelength of 400-750 nm), within which vision is possible.
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resultant
A vector that is the vector sum of multiple vectors
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solid
A substance in the fundamental state of matter that retains its size and shape without need of a container (as opposed to a liquid or gas).
##### Appears in these related concepts:
superimpose
To place an object over another object.
##### Appears in these related concepts:
superposition
The summing of two or more field contributions occupying the same space.
##### Appears in these related concepts:
wave
A moving disturbance in the energy level of a field.
##### Appears in these related concepts:
work
A measure of energy expended in moving an object; most commonly, force times displacement. No work is done if the object does not move.