Waves are disturbances in water, air, or another substance, or in a field of force. Waves on water are perhaps the most familiar example, but sound and light also travel in waves. Waves carry energy from place to place and can also transmit information.
It is easy to generate a wave. If you throw a stone into a large, still pond, a series of disturbances will travel outward from the point where the stone enters the water. The disturbances will be expanding, ring-shaped waves, all with the same center—the entry point of the stone.
Energy produces the disturbances, and the disturbances carry energy. The movement of the stone from your hand to the water’s surface carries both matter—that is, the stone—and energy. As the stone penetrates the surface, energy carried by the stone displaces some water. The water closest to where the stone entered rises and then falls, producing a ring-shaped disturbance with the stone’s entry point at its center. The falling water displaces other nearby water, causing another sequence of rising and falling, which in turn causes still others. As a result, a wave travels outward as an expanding ring. But the water closest to the stone’s entry point has enough energy to rise and fall again several times. So a series of circular waves travel outward.
Another simple wave experiment involves a rope. Ask two people to hold the ends of the rope. When one person moves an end of the rope up and down sharply, energy moves from that person’s hand and travels through the rope. The person at the other end will feel the incoming energy move his or her hand. As the energy passes through the rope, the rope moves up and down, but does not move forward.
Scientists call the material in which waves travel the wave medium. Water is the medium for water waves, and the rope is the medium for rope waves. Waves cause little overall displacement of their medium unless the disturbances become unusually large. For example, water waves travel horizontally across the surface of the water, but usually almost all the motion of the water itself is vertical.
Light waves and other kinds of electromagnetic waves are disturbances in a nonmaterial medium—fields of electric and magnetic force. In physics, a field is a region of space in which a particular kind of force can be felt. For example, a magnetic field is a region in which the force of magnetism can be felt. A magnet produces such a field in the space around itself.
Radio waves are another kind of electromagnetic wave. These waves can be controlled to carry information. Radio and television broadcasts and cellular telephone messages travel through the air on radio waves.
Waves may be one-, two-, or three-dimensional. Rope waves travel along the length of the rope and are one-dimensional. Water waves generated by a falling stone form circles that spread out over the surface of the pond. These waves are two-dimensional. Sound originates at a point and travels equally strongly in all directions. Sound waves are spherical and three-dimensional.
Types of waves
There are two main types of waves, longitudinal and transverse, depending on the motion of the medium. In transverse waves, the medium moves perpendicular to the direction in which the wave travels. For example, a rope wave is a transverse wave because the rope moves up and down while the wave moves from one end of the rope to the other. Light waves and other electromagnetic waves are also transverse waves.
Waves that travel in the same direction that the medium moves are longitudinal waves. Longitudinal waves require a medium that can be compressed. You can generate a longitudinal wave in a stretched spring by squeezing and then releasing a few coils at one end of the spring. Sound waves and some seismic (earthquake-generated) waves are longitudinal waves.
Measuring waves
Waves are usually measured by their amplitude, height, frequency, and wavelength. We can determine the size of a wave by measuring the maximum distance that the wave medium moves from its usual position. For example, as rope waves travel through a rope, the rope forms a wavy line. The highest point on the line is called the wave crest, and the lowest point is the wave trough. The vertical distance from the crest to the level of the rope at rest is the wave amplitude.
The distance from the usual position of the rope to the bottom of the trough may differ from the amplitude. When water waves move toward shore, for example, their crests become higher and sharper, while their troughs become shallower and wider. In this situation, the size of a wave is measured by wave height, the vertical distance from the crest to the trough.
The number of waves that pass a given point in a given time is called the frequency of the series of waves. By moving the free end of the rope up and down faster, for example, we increase the frequency of the waves because more waves pass any point in a given time.
As the frequency is increased, the distance between the two adjacent wave crests or troughs is shortened. This distance is called wavelength. The shorter the wavelength, the higher the frequency.
How waves move
The simplest waves rise and fall with a fixed frequency and wavelength. Such waves are called simple harmonic waves. But most wave motions are combinations of several waves with different frequencies and wavelengths.
Combined waves are either nondispersive or dispersive. In a nondispersive wave, all the individual waves travel at the same speed, regardless of their frequency. Water waves in shallow water are nondispersive. Their speed depends only on the square root of the depth of the water. The higher this number is, the more rapid are the waves. In a dispersive wave, the individual waves travel at speeds that depend on their frequencies. The longer wavelengths tend to race ahead of the shorter ones, making the combined wave spread out. This spreading occurs in much the same way that a tight pack of racehorses spreads out as the faster horses pull out in front of the slower ones. Water waves in deep water are dispersive.
Other factors tend to compress waves. When ocean waves approach the shore, for example, the shallow bottom may slow down the waves and cause them to pile up, much as a loose pack of racehorses bunches up when the front-runners suddenly encounter rough terrain.
Under the right conditions, the effects of dispersion and compression balance out, producing an unusual wave called a solitary wave or soliton. A soliton has a single humplike crest but no trough. Solitons do not readily disperse, but instead keep their size and shape much longer than ordinary waves do. Solitons often form in shallow water channels.
Most waves, such as water waves, travel from one place to another. These waves are called progressive waves. Under certain conditions, however, waves can be trapped in a medium. For example, if a rope is held at both ends and plucked, the energy in the waves cannot leave the rope at either end. This trapping of waves results in patterns of vibration called standing waves. Standing waves vibrate at a certain frequency but do not move along the medium. The vibrations of guitar strings and violin strings are standing waves. Standing waves can also occur on a surface, such as a drumhead, or in an enclosed space, such as a pipe of a pipe organ.
Changes in wave motion
Wave motion changes when waves move from one medium to another and when one wave meets another.
Refraction and reflection.
Refraction is a change in wave direction when waves pass from one medium into another. When waves leave one medium and enter another, some of the energy in the waves is reflected and some is transmitted to the new medium. The amount of energy reflected and transmitted depends on the angle of incidence—that is, the angle at which the incoming wave strikes the medium. It also depends on the density and other properties of the two mediums. The direction taken by the transmitted wave, called the refracted wave, also depends on the angle of incidence and the characteristics of the two mediums.
Diffraction
is the spreading out of waves as they pass through an opening or by the edge of an obstacle. An expanding ring of water waves moves away from a stone dropped into a pond. As the ring becomes larger, any short part of the wave front (the outside edge of the ring) becomes a nearly straight line. However, if the wave front passes through a small slit in a barrier, the wave coming out the other side will spread out from the slit in a curve.
Diffraction occurs because each point on the surface of a wave is the source of small, curved waves called wavelets that move outward in all directions. The wavelets along the front combine to make the straight-line wave front. But the slit allows only a few wavelets through. The wavelets on either side of the slit are blocked, so the wave front is no longer straight.
Interference.
Where the crests of two waves with the same frequency pass the same point at the same time, the waves are said to be in phase with each other—that is, their crests and troughs coincide. The two waves combine to produce a single wave with a larger amplitude. The amplitude of the combined wave equals the sum of the amplitudes of the two original waves. Scientists say that constructive interference takes place.
If the crest of one wave passes a point at the same time as the trough of the other, however, the waves combine to produce a wave with a smaller amplitude. The amplitude of the combined wave equals the difference between the amplitudes of the original waves. Destructive interference occurs.
