TYPES OF WAVES
Waves are divided into types according to the direction of the displacements in relation to the direction of the motion of the wave itself. If the vibration is parallel to the direction of motion, the wave is known as a longitudinal wave. The longitudinal wave (or P wave) is always mechanical because it results from successive compressions (state of maximum density and pressure) and rarefactions (state of minimum density and pressure) of the medium. Sound waves typify this form of wave motion. The successive compressions and rarefactions (i.e., oscillation) of the particles of matter (or the medium) help the longitudinal wave to travel. When a person speaks, the molecules of air (or the medium) oscillate to produce a longitudinal wave that can reach listeners' ears (see figure 7.1).
Another type of wave is the transverse (or S wave) wave, in which the vibrations are at right angles (90 degrees) to the direction of motion. A transverse wave may be mechanical, such as the wave projected in a taut string that is subjected to a transverse vibration; or it may be electromagnetic, such as light, X ray, or radio waves. Some mechanical wave motions, such as waves on the surface of a liquid, are combinations of both longitudinal and transverse motions, resulting in the circular motion of liquid particles (see figure 7.2).
For a transverse wave, the wavelength is the distance between two successive crests or troughs. For longitudinal waves, it is the distance from compression to compression or rarefaction to rarefaction. The frequency of the wave is the number of vibrations per second. The velocity of the wave, which is the speed at which it advances, is equal to the wavelength times the frequency. The maximum displacement involved in the vibration is called the amplitude of the wave. The sequence of a trough and a crest is called a cycle. Frequency is the technical term used to refer to the number of cycles that occur in a given period of time (usually one second). Hz (hertz) is the measurement unit that is used to measure wave frequencies. One Hz is equal to one cycle per second.
