From sound and light to ocean waves and seismic waves, we’re surrounded by waves. Understanding musical instruments, cell phones, or lasers requires a knowledge of waves. With this chapter we shift our focus from the particle model to a new wave model that emphasizes those aspects of wave behavior common to all waves.

The wave model is built around the idea of a traveling wave, which is an organized disturbance traveling with a well-defined wave speed. We’ll begin our study of traveling waves by looking at two distinct wave motions.

We can also classify waves on the basis of what is “waving”:

1. Mechanical waves travel only within a material medium, such as air or water. Two familiar mechanical waves are sound waves and water waves.

2. Electromagnetic waves, from radio waves to visible light to x rays, are a self-sustaining oscillation of the electromagnetic field. Electromagnetic waves require no material medium and can travel through a vacuum.

The medium of a mechanical wave is the substance through or along which the wave moves. For example, the medium of a water wave is the water, the medium of a sound wave is the air, and the medium of a wave on a stretched string is the string. A medium must be elastic. That is, a restoring force of some sort brings the medium back to equilibrium after it has been displaced or disturbed. The tension in a stretched string pulls the string back straight after you pluck it. Gravity restores the level surface of a lake after the wave generated by a boat has passed by.

As a wave passes through a medium, the atoms of the medium—we’ll simply call them the particles of the medium—are displaced from equilibrium. This is a disturbance of the medium. The water ripples of FIGURE 16.1 are a disturbance of the water’s surface. A pulse traveling down a string is a disturbance, as are the wake of a boat and the sonic boom created by a jet traveling faster than the speed of sound. The disturbance of a wave is an organized motion of the particles in the medium, in contrast to the random molecular motions of thermal energy.

FIGURE 16.1 Ripples on a pond are a traveling wave.