Waves

Next, have one boy throw a baseball to the other. The catcher feels some of the energy used in throwing the ball. But unlike the wave, the matter--that is, the ball--moves forward. As the ball moves, it carries the energy with it.

Waves on a rope or water are familiar examples of waves, but many other waves move around us all the time. For example, the sound of people speaking travels to our ears as waves. Radio and television programs travel to our homes as waves.

Many kinds of waves travel on or in a material. Scientists call the substance through which the waves travel the wave medium. For waves on a rope, the rope is the medium. Ocean waves travel on the surface of the water and seismic (earthquake) waves travel through the earth. Some waves do not need to travel through a material medium. For example, the electromagnetic waves that carry light, radio, and television signals can travel through a vacuum. The medium for these waves is an electric and magnetic field (area containing lines of force).

By moving the free end of the rope up and down faster, you make more waves on the rope. You have increased the frequency of the waves because more waves pass any point in one second. But no matter how fast or how high you make the rope move, you cannot make the waves travel faster. The speed of a wave does not depend on either its amplitude or its frequency. The speed of the wave depends only on the stiffness and density (mass of a unit of volume) of the medium. For example, a tightly stretched rope will have a higher wave speed than a slack rope. Waves will also travel faster on a light, less dense rope that is stretched the same amount as a heavy rope.

When you increase the frequency of the waves, you also shorten the distance between the crests or troughs. Scientists call this distance the wavelength. You can find the wavelength by dividing the frequency into the wave speed.

The frequency of sound waves is also called the pitch. The pitch of a note produced by a stringed instrument can be raised by tightening or shortening the string. Strings designed to produce lower frequencies generally are heavier, longer, and less tightly stretched.

Transverse waves cause the medium to move up and down while the wave travels along it. Waves that move this way are called transverse waves, because the motion of the medium is perpendicular to the motion of the waves. Rope waves are transverse. Other examples of transverse waves include electromagnetic waves such as light, and water waves. If the rope moves vertically or horizontally, the waves have a vertical or horizontal polarization. That is, the medium vibrates in only one direction. Only transverse waves can have polarization.

Longitudinal waves, also called compressional waves, travel in the same direction that the medium moves. For example, these waves are made in a stretched spring by compressing a few coils at one end and then releasing them. Sound waves and some earthquake waves are longitudinal waves.

Traveling and standing waves. In the previous examples with waves on a rope, the waves have traveled from one end of the rope to the other. These waves are traveling waves. However, under certain conditions, waves may be trapped in a medium. For example, if a string is held at both ends and plucked, the energy in the waves cannot leave the string at either end. This creates patterns called standing waves. The size of the space in which the waves are confined determines the wavelength the waves can have. Standing waves can exist on a surface such as a drumhead, or within an enclosed space, such as a room. The possible wavelengths are still determined by the size of the medium.

Reflection and refraction. When waves leave one medium and enter another, some of the energy in the waves is reflected and some is refracted (transmitted) into the new medium. The amount of energy that is reflected and refracted depends on the angle at which the incident (incoming) waves strike the new medium. The larger the angle between the path of the waves and an imaginary line perpendicular to the surface of the new medium, the more the waves will be reflected.

The amount of reflection and refraction also depends on certain properties of the two mediums. For example, reflection and refraction of sound waves depends on the density of the two mediums and the speed of sound in them. If the two properties are nearly the same, most of the sound will be refracted into the new medium. If they are different, most of the sound will be reflected. Air is much less dense than the ground and carries sound much more slowly. Consequently, most of the energy in sound waves is reflected from the ground.

Diffraction. An expanding ring of waves moves away from a stone dropped into a still pond. As the ring becomes larger, any short part of the wave front (the outside edge of the ring) becomes a nearly straight line. But if the wave front passes through a small slit in a barrier, the wave coming out on the other side will not form a straight line. Instead, it will spread out from the slit in a curved line.

The changing of the straight wave front into a curved wave is called diffraction. Diffraction occurs because each point on the wave front is a source of a tiny curved wave called a wavelet. The wavelets along the front combine to make the straight wave. But the slit lets only a few wavelets pass through. The wavelets on either side are cut off and the front is no longer straight.

Interference. Where the crests of two waves with the same frequency pass a given point at the same time, the waves are in phase. But, if the crest of one wave passes the point at the same time as the trough of the other, one wave is half a wavelength ahead of the other. Scientists measure the difference in phase between two waves in degrees. They multiply the number of degrees in a circle (360) by the fraction of the wavelength between the two waves. In this case, the waves are 180º out of phase.

Waves of the same frequency make each other stronger where they are in phase and cancel each other where they are 180º out of phase. Scientists say the two sources interfere with each other. The waves will travel away stronger in some directions and weaker in others.

Wave theory. The ideas that apply to light, sound, and other waves also apply to the tiny parts of atoms. Scientists have discovered that electrons, neutrons, and protons, which are usually thought of as particles, sometimes behave like waves. Their waves are called matter waves. The wave theory of atomic particles has given scientists a greater understanding of the structure of atoms and their nuclei. See Physics (Einstein and relativity); Quantum mechanics.

Contributor: L. Wallace Dean, Ph.D., Former Engineer, Pratt and Whitney Aircraft.

Related articles include:

Electromagnetic Waves; Interference; Laser; Light; Polarized Light; Radio; Reflection; Refraction; Sound; Television.

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