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9: Waves

In physics a wave is a disturbance or oscillation that travels through space accompanied by a transfer of energy, and may be propagated with little or no net motion of the medium involved. In this section we will consider mechanical waves, in which the particles in a material are oscillating. Examples are the waves in the sea, the wave in the crowd at a stadium, and sound. Later on we will encounter electromagnetic waves in which electric and magnetic fields are oscillating, and which can travel through vacuum. Examples are light and radio signals. In quantum mechanics, we will also encounter what are sometimes referred to as matter waves, where fundamental objects that we usually think of as particles, such as electrons and protons, can also be considered as waves. Finally, recently gravitational waves were discovered, which are vibrations of space time itself.

By observing a particle, we know in which direction it moves at any given time. However, as I just stated,the particles in a mechanical wave have no, or almost no, net motion as the wave passes. The wave does have a well-defined direction though: the direction in which energy is transferred. Some waves spread out uniformly, such as a sound wave emanating from a point source. Others are restricted in their motion by the properties of the material they travel in, such as a wave in a string, or by boundary conditions, such as the end of that string. For waves that move (predominantly) in one direction, we can distinguish two fundamental types, illustrated in Figure 9.1.1. The first type is the case that the particles oscillate in the same direction as the wave is moving (Figure 9.1.1a), which we call a longitudinal wave; sound is an example. The second case is that the particles oscillate in a direction perpendicular to the wave motion, which we call a transverse wave (Figure 9.1.1b), of which the waves in a pond are an example.

• 9.1: Sinusoidal Waves
Probably the simplest kind of wave is a transverse sinusoidal wave in a one-dimensional string. In such a wave each point of the string undergoes a harmonic oscillation.
• 9.2: The Wave Equation
As with all phenomena in classical mechanics, the motion of the particles in a wave, for instance the masses on springs in Figure 9.1.1, are governed by Newton’s laws of motion and the various force laws. In this section we will use these laws to derive an equation of motion for the wave itself, which applies quite generally to wave phenomena.
• 9.3: Solution of the One-Dimensional Wave Equation
The one-dimensional wave Equation 9.2.6 has a surprisingly generic solution, due to the fact that it contains second derivatives in both space and time.
• 9.4: Wave Superposition
The wave equation 9.2.6 is linear in the function we’re interested in, the displacement u(x,t) . This simple mathematical statement has important consequences, because it means that if we know any set of solutions, we can create more solutions by making linear combinations of them.
• 9.5: Amplitude Modulation
The general solution to the wave equation allows for many more interesting wave shapes. An important, and often encountered one is where the wave itself is used as the medium, by changing the amplitude over time.
• 9.6: Sound Waves
So far, we mostly considered transversal waves, which include waves in strings and waves on the surface of a pond, and are easily visualized. Longitudinal waves, on the other hand, are somewhat harder to draw, but easily heard - as sound is the prime example of a longitudinal wave.
• 9.7: The Doppler Effect
The Doppler effect is a physical phenomenon that most people have experienced many times: when a moving source of sound (say an ambulance, or more exactly its siren) is approaching you, its pitch sounds noticeably higher then after it passed you by and is moving away. The effect is due to the fact that the observed wavelength (and therefore frequency / pitch) of sound corresponds to the distance between two points of equal phase (i.e., two sequential wavefronts).
• 9.E: Waves (Exercises)

Thumbnail: Surfer at Mavericks, one of the world's premier big wave surfing locations. (Surfer: Andrew Davis).Image used with permission (CC SA-BY 2.0; Shalom Jacobovitz).