Waves

WAVES

So far, oscillations have been clearly discussed but we have not talked about waves. What is a wave? This is a disturbance that propagates from the place it was created. For water, the wave can be seen; for sound waves, the wave can be seen as air pressure. For earthquakes, it can be felt as earth moving. A wave in water is an up and down disturbance of the water surface. There will be peaks and troughs of the wave. The time for a complete peak to peak motion is the wave’s period T with the frequency being 1 divided by the period. The wave’s velocity v is the speed at which the wave moves across a surface or in air. This is also referred to as the propagation velocity. The water itself in a wave does not move forward, it just goes up and down with a certain energy being propagated. The wavelength, identified by the Greek letter lambda is the peak to peak distance.

The wavelength, which can also be described as the distance between adjacent identical parts of the wave, will be a distance that is parallel to the direction of the propagation. The speed will be the time of one period or lambda divided by T. It can also be described as the frequency multiplied by the wavelength. Figure 103 shows these relationships:

Figure 103.

Velocity can be speed of water waves, speed of light, or speed of sound. You can get the velocity by knowing, for example the time between wave crests (which is the period) and the wavelength.

Waves that travel like ocean waves will propagate in the horizontal direction, while the surface is disturbed in the vertical direction. These ocean waves are called transverse waves or shear waves. The disturbance is perpendicular to the direction of propagation. In contrast, there can be a longitudinal wave or compression wave, in which the disturbance is parallel to the direction of propagation. In a longitudinal wave, the size of the disturbance is an amplitude of X, which is independent of the speed of propagation. A spring that is set in a wave fashion is a longitudinal or compression wave as is shown in figure 104:

Figure 104.

Waves may be transverse, longitudinal, or a combination of these. Water waves in the ocean are a combination of transverse and longitudinal waves. Electromagnetic waves and light waves are all transverse or shear waves, while sound waves in air and water are considered longitudinal waves. Sound waves in solids are both longitudinal and transverse. Earthquake waves under the Earth’s surface will have both longitudinal and transverse components. They will propagate at different speeds.