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Waves particle theory

  1. May 5, 2012 #1
    do all particles in a wave have the same speed? they do right, else their frequency would be different, and frequency is only dependent on the freqeuncy of the source. but then again how can the speed of particles in a wave be the same , actually? logically speaking, a particle going up and coming down has different velocity, right? also, the wavelength decreases as the speed decreases in shallow water as compared to deep water. but why is this so? why does the speed and wavelength of a wave decrease in shallow water?

    Another question that's related: why is it that when we place a plastic sheet at an angle to incoming plane waves in ripple tank, and the water becomes shallow, that the change in speed of the waves will cause the waves to bend?
    all help will be greatly appreciated. and i would prefer to not have a mathematical outlook on the situation, as i've only learnt the basic wave formulas.
    Last edited: May 5, 2012
  2. jcsd
  3. May 6, 2012 #2


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    Staff: Mentor

    A wave of what?
    Waves on water? No, the water particles have velocities and directions which are completely different from the wave propagation.
    Sound in a medium? No
    Light? In vacuum the photons all have the same speed, in a medium it usually depends on the wavelength.

    Don't mix the oscillation of particles in some direction with the velocity of a wave.

    I think this needs a simulation of the water as a fluid. However, the individual particles in the water do not matter, you can treat water as a continuous fluid without any particles.

    Your last question is related to refraction, maybe the wikipedia article helps. The different velocity of the wave at different positions is important here.
  4. May 6, 2012 #3

    I meant a sound wave. Why don't the particles in a sound wave travel at the same speed? You mean to say that the velocity of the particles is different from the velocity of the wave? Shouldn't it be the same? Isn't the wave created by the particles themselves ?

    I don't get how treating water as a fluid helps me to understand the change in wavelength and speed.....
  5. May 7, 2012 #4


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    Celluhh, You cover a lot of areas with your questions in your Opening Post! Here are f few excerpts from a few sites. Hopefully you discover your answers:

    “In a longitudinal wave the particle displacement is parallel to the direction of wave propagation. The animation below shows a one-dimensional longitudinal plane wave propagating down a tube.

    In a transverse wave the particle displacement is perpendicular to the direction of wave propagation.

    Water waves are an example of waves that involve a combination of both longitudinal and transverse motions.”

    “In fluid dynamics, dispersion of water waves generally refers to frequency dispersion, which means that waves of different wavelengths travel at different phase speeds. Water waves, in this context, are waves propagating on the water surface, and forced by gravity and surface tension. As a result, water with a free surface is generally considered to be a dispersive medium.

    Surface gravity waves, moving under the forcing by gravity, propagate faster for increasing wavelength. For a given wavelength, gravity waves in deeper water have a larger phase speed than in shallower water. In contrast with this, capillary waves only forced by surface tension, propagate faster for shorter wavelengths.”
    http://en.wikipedia.org/wiki/Dispersion_(water_waves [Broken])

    “Diffraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, X-rays and radio waves.”

    If you find "no joy", then come right back here and post your questions or doubts.
    Last edited by a moderator: May 6, 2017
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