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# Wave Interference

## Discusses what happens when waves collide with each other.

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Wave Interference

When raindrops fall into still water, they create tiny waves that spread out in all directions away from the drops. What happens when the waves from two different raindrops meet? They interfere with each other.

### When Waves Meet

When two or more waves meet, they interact with each other. The interaction of waves with other waves is called wave interference . Wave interference may occur when two waves that are traveling in opposite directions meet. The two waves pass through each other, and this affects their amplitude. Amplitude is the maximum distance the particles of the medium move from their resting positions when a wave passes through. How amplitude is affected by wave interference depends on the type of interference. Interference can be constructive or destructive.

### Constructive Interference

Constructive interference occurs when the crests, or highest points, of one wave overlap the crests of the other wave. You can see this in the Figure below . As the waves pass through each other, the crests combine to produce a wave with greater amplitude. You can see an animation of constructive interference at this URL: http://phys23p.sl.psu.edu/phys_anim/waves/embederQ1.20100.html

### Destructive Interference

Destructive interference occurs when the crests of one wave overlap the troughs, or lowest points, of another wave. The Figure below shows what happens. As the waves pass through each other, the crests and troughs cancel each other out to produce a wave with zero amplitude. You can see an animation of destructive interference at this URL: http://phys23p.sl.psu.edu/phys_anim/waves/embederQ1.20200.html

### Standing Waves

Waves may reflect off an obstacle that they are unable to pass through. When waves are reflected straight back from an obstacle, the reflected waves interfere with the original waves and create standing waves . These are waves that appear to be standing still. Standing waves occur because of a combination of constructive and destructive interference. You can see animations of standing waves at the URLs below.

Q : How could you use a rope to produce standing waves?

A : You could tie one end of the rope to a fixed object, such as doorknob, and move the other end up and down to generate waves in the rope. When the waves reach the fixed object, they are reflected back. The original waves and the reflected waves interfere to produce a standing wave. Try it yourself and see if the waves appear to stand still.

### Summary

• Wave interference is the interaction of waves with other waves.
• Constructive interference occurs when the crests of one wave overlap the crests of the other wave, causing an increase in wave amplitude.
• Destructive interference occurs when the crests of one wave overlap the troughs of the other wave, causing a decrease in wave amplitude.
• When waves are reflected straight back from an obstacle, the reflected waves interfere with the original waves and create standing waves.

### Vocabulary

• standing wave : Wave appearing to stand still that forms when a wave and its reflected wave interfere.
• wave interference : Interaction of waves with other waves.

### Practice

Review wave interference at the following URL. Then do the Check Your Understanding problem at the bottom of the Web page. Be sure to check your answers. http://www.physicsclassroom.com/class/waves/u10l3c.cfm

### Review

1. What is wave interference?
2. Create a table comparing and contrasting constructive and destructive interference.
3. What are standing waves? How do they form?

### Vocabulary Language: English

standing wave

standing wave

Wave appearing to stand still that forms when a wave and its reflected wave interfere.
wave interference

wave interference

Interaction of waves with other waves.