UNDERSTANDING WAVE MOTION:

Dan Russell - A library of physically and mathematically correct sound animations, accompanied by explanatory text, that illustrate complicated phenomena involving waves and vibration in a manner that aids student understanding.

LONGITUDINAL WAVES (sound): Sound waves are 'longitudinal' waves:

Please see this site for highly detailed and accurate information about sound waves Dan Russell - Sound wave animation library.

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In a longitudinal wave the particle displacement is parallel to the direction of wave propagation. The animation (Fig 1.) shows a one-dimensional longitudinal plane wave propagating down a tube. The particles do not move down the tube with the wave; they simply oscillate back and forth about their individual equilibrium positions. Pick a single particle and watch its motion. The wave is seen as the motion of the compressed region (ie, it is a pressure wave), which moves from left to right.

Fig 1. Longitudinal Waves - particles & propagation

The following animation (Fig 2.) shows the difference between the oscillatory motion of individual particles and the propagation of the wave through the medium. The animation also identifies the regions of compression and rarefaction.

Fig 2. Longitudinal Waves - particles & propagation

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TRANSVERSE WAVES (light - electromagnetic):

Light waves are an example of transverse waves - Sound waves are not transverse waves: In a transverse wave the particle displacement is perpendicular to the direction of wave propagation. The animation below (Fig 3.) shows a one-dimensional transverse plane wave propagating from left to right. The particles do not move along with the wave; they simply oscillate up and down about their individual equilibrium positions as the wave passes by. Pick a single particle and watch its motion.

Fig 3. Transverse Waves - particle motion

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WATER WAVES (liquids):

Water waves are an example of waves that involve a combination of BOTH longitudinal and transverse motions (Fig 4.). As a wave travels through the waver, the particles travel in clockwise circles. The radius of the circles decreases as the depth into the water increases. The animation below shows a water wave travelling from left to right in a region where the depth of the water is greater than the wavelength of the waves. I have identified two particles in yellow to show that each particle indeed travels in a clockwise circle as the wave passes.

Fig 4. Water Waves - particle rotation

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RAYLEIGH SURFACE WAVES (various):

Another example of waves with both longitudinal and transverse motion may be found in solids as Rayleigh surface waves. The particles in a solid, through which a Rayleigh surface wave passes, move in elliptical paths, with the major axis of the ellipse perpendicular to the surface of the solid. As the depth into the solid increases the “width” of the elliptical path decreases.

Rayleigh waves are different from water waves in one important way. In a water wave all particles travel in clockwise circles. However, in a Rayleigh surface wave, particles at the surface trace out a counter-clockwise ellipse, while particles at a depth of more than 1/5th of a wavelength trace out clockwise ellispes.

The animation below (Fig 5.) shows a Rayleigh wave travelling from left to right along the surface of a solid. I have identified two particles in yellow to illustrate the combines counter-clockwise + clockwise motion as a function of depth.

The Rayleigh surface waves are the waves that cause the most damage during an earthquake.

Fig 5. Rayleigh Waves - elliptical motion

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Salford University - Detailed and advanced information about sound waves.