Waves Study Guide
This lesson studies the general characteristics of waves such as frequency, speed, and wavelength; sound waves and propagation in a medium; and electromagnetic waves and electromagnetic spectrum.
Characteristics of a Wave
The physical appearance of a water wave or of a string that's fixed at one end but pulsed vertically at the other is very similar to the time dependence of the displacement we have studied in the previous lesson. The quantities we have defined there will be reused in the case of waves.
Waves are disturbances produced in matter by the interaction with a source that supplies an energy. This energy comes from the wind in the case of the ocean waves, from an earthquake in the case of tsunami waves, or from your finger in the case of guitar strings.
Also, a wave has the disturbance propagated through the material with no bulk flow of matter; therefore, a wave is not the motion of masses of water but simply the propagation of the energy from one position to another. The motion of a particle of water in a wave is actually circular, as shown in Figure 17.1.
The water particle is seen to move up and down and not in the direction of propagation of the wave. In conclusion, waves have some characteristics that are worth summarizing: They represent a disturbance in the matter, they carry energy through the matter, and they do not involve bulk flow of matter.
The way the water particles travel while acted on by a wave is the more complex part of the idea of a wave. We can break the behavior down into two more simple types of propagation. According to the relative direction of the disturbance with respect to the propagation of the wave, there are longitudinaland transversal waves.
Longitudinal waves have the disturbance and the wave propagation parallel to each other, such as in the case of a slinky set on a table and pushed back and forth. Regions along the slinky will be compressed, and others will be stretched. See Figure 17.2.
Transversal waves have the disturbance and the wave propagation perpendicular to each other, such as in the case of a slinky set on a table and pulsed up and down.
If you now take a string and fix it at one end and leave the other one free as seen in Figure 17.3, and at the free end, you start applying an up and down pulse, the string will be disturbed in a vertical direction while the wave will propagate to the right as in Figure 17.4.
As one can see, the waves discussed here have the same cyclic behavior as encountered previously in the simple harmonic motion. We will call these waves periodic waves, and we will define their period, frequency, amplitude, and speed, as we did for harmonic motion. See Figures 17.5 and 17.6 for graphic representations of these quantities.
The period will be the time it takes a point acted on by the disturbance to repeat motion, and it is the inverse of the frequency (which represents the number of complete cyclic motions that a point describes in a second). The wavelength is the distance between two identical positions that the wave has reached. And the amplitude is the maximum displacement of the particle relative to the equilibrium position.
The speed of the wave is defined as the ratio of the wavelength to the period:
But because the period and frequency are inversely proportional, we have also:
v = f · λ
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