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5.2.1: Interference

  • Page ID
    18973
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    5.9.jpg
    Figure 5.9: In-phase and out-of-phase waves

    Besides wavelength (λ) and frequency (ν), an amplitude and a phase characterize all waves. Amplitude (A) refers to the height of a wave. Phase refers to whether a wave is moving up or down at a particular time. If two waves move up and down at same times, they are in phase; if not, they are out of phase.

    When two waves with the same wavelength travel in the same direction simultaneously, they interfere with each other. Their amplitudes may add, cancel, or be somewhere between. The nature of the interference depends on the wavelengths, amplitudes, and phases of the two waves. Light waves passing through crystals can have a variety of wavelengths, amplitudes, and phases affected by atomic structure in different ways. They yield interference phenomena, giving minerals distinctive optical properties.

    In Figure 5.9a, two in-phase waves of the same wavelength are going in the same direction. If we could measure the intensity of the two waves together, we would find that their amplitudes have added. When waves are in phase, no energy is lost; this is constructive interference. In contrast, Figure 5.9b shows two waves that are slightly out of phase, and Figure 5.9c shows two waves that are completely out of phase.

    When waves are out of phase, wave peaks and valleys do not correspond. If they are completely out of phase, the peaks of one wave correspond to the valleys of the other. Consequently, addition of out-of-phase waves can result in destructive interference, a condition in which the waves appear to “consume” some or all of each other’s energy. (The First Law of Thermodynamics tells us that energy cannot disappear, so if two waves appear to cancel each other it means that the energy is going in another direction.) For perfect constructive or destructive interference to occur, waves must be of the same wavelength but they may have different amplitudes (as shown in Figure 5.9c). Interaction of waves with different wavelengths is much more complicated.


    This page titled 5.2.1: Interference is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Dexter Perkins via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.