Wave motion is a disturbance that moves from place to place in some medium, carrying energy with it. Probably the most familiar example of wave motion is the action of water waves. A boat at rest on the ocean moves up and down as water waves pass beneath it. The waves appear to be moving toward the shore. But the water particles that make up the wave are actually moving in a vertical direction. The boat itself does not move toward the shore or, if it does, it's at a much slower rate than that of the water waves themselves.
The energy carried by a water wave is obvious to anyone who has watched a wave hit the shore. Even small waves have enough energy to move bits of sand. Much larger waves can, of course, tear apart the shore and wash away homes.
Two types of waves exist: transverse and longitudinal. A transverse wave is one that causes the particles of the surrounding medium to vibrate in a direction at right angles to the direction of the wave. A water wave is an example of a transverse wave. As water particles move up and down, the water wave itself appears to move to the right or left.
Amplitude: The maximum displacement (difference between an original position and a later position) of the material that is vibrating. Amplitude can be thought of visually as the highest and lowest points of a wave.
Condensation: A region of space with a higher-than-normal density.
Crest: The highest point reached by a wave.
Frequency: The number of wave crests (or wave troughs) that pass a given point per unit of time (usually per second).
Longitudinal wave: A wave that causes the particles of the surrounding medium to vibrate in the same direction as that in which the wave is moving.
Rarefaction: A region of space with a lower-than-normal density.
Transverse wave: A wave that causes the particles of the surrounding medium to vibrate in a direction at right angles to the direction of the wave motion.
Trough: The lowest point reached by a wave.
Wavelength: The distance between any two adjacent wave crests (wave crests that are next to each other) or any two adjacent wave troughs in a wave.
A longitudinal wave is one that causes the particles of the surrounding medium to vibrate in the same direction as that in which the wave is moving. A sound wave is an example of a longitudinal wave. A sound wave is produced when the pressure in a medium is suddenly increased or decreased. That pressure change causes pulses of rarefactions and condensations to spread out away from the source of the sound. A rarefaction is a region of space with a lower-than-normal density; a condensation is a region with a higher-than-normal density. The sound wave travels from one place to another as particles vibrate back and forth in the medium in the same direction as the sound wave.
Any wave can be fully characterized by describing three properties: wavelength, frequency, and amplitude. Like any wave, a water wave appears to move up and down in a regular pattern. The highest point reached by the wave is known as the wave crest; the lowest point reached is the wave trough (pronounced trawf).
The distance between any two adjacent (next to each other) wave crests or any two adjacent wave troughs is known as the wavelength of the wave. The wavelength is generally abbreviated with the Greek letter lambda, λ . The number of wave crests (or wave troughs) that pass a given point per unit of time (usually per second) is known as the frequency of the wave. Frequency is generally represented by the letter f. The highest point reached by a wave above its average height is known as the amplitude of the wave. The speed at which a wave moves is the product of its wavelength and its frequency, or, v = λ f.
Two kinds of waves most commonly encountered in science are sound waves and electromagnetic waves. Electromagnetic radiation includes a wide variety of kinds of energy, including visible light, ultraviolet light, infrared radiation, X rays, gamma rays, radar, microwaves, and radio waves. As different as these forms of energy appear to be, they are all alike in the way in which they are transmitted. They travel as transverse waves with the same velocity, about 3 × 10 10 centimeters (1.2 × 10 10 inches) per second, but with different wavelengths and frequencies.
Waves have many interesting properties. They can reflect from surfaces and refract, or change their direction, when they pass from one medium into another. An example of reflection is the light we observe that bounces off an object, allowing us to see that object. An example of refraction is the apparent dislocation of objects when they are placed underwater.
Waves also can interfere, or combine, with each other. For example, two waves can reach a particular point at just the right time for both to disturb the medium in the same way. This effect is known as constructive interference. Similarly, destructive interference occurs when the disturbances of different waves cancel each other out. Interference can also lead to standing waves—waves that appear to be stationary. The medium is still disturbed, but the disturbances are oscillating in place. Standing waves can occur only within confined regions, such as in water in a bathtub or on a guitar string that is fixed at both ends.