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11.5: Wave Interference

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    Figure 11.5.1

    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.

    Diagram illustrating constructive interference
    Figure 11.5.2

    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.

    Diagram illustrating destructive interference
    Figure 11.5.3

    In shallow ponds, the depth of the water influences the wave speed. Use the Stow Lake simulation below to make waves of different speeds. These will display different interference patterns. Try pausing the simulation to analyze the wave in the graph and find a spot where the wave pattern is maximized and where it is zero to learn more about wave interference:

    Interactive Element

    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.

    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.

    Have you ever done “the wave” at a sporting event? Well, then you have helped to create a wave. Learn more about waves and wave interference by launching the Stadium Wave simulation below:

    Interactive Element

    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.

    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?

    Additional Resources

    Study Guide: Waves Study Guide

    Real World Application: Look Ma, No Hands!

    Videos:

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