Waves, rhythmic motions by which energy, including light and sound, is transmitted through matter or space. Only electromagnetic waves, such as radio, light, and infrared waves, can pass through a vacuum. Other kinds of waves can travel only through matter, such as air or water.
All vibrations produce waves. Vibrating vocal cords, for example, produce sound waves; and the vibrations, or oscillations, of an electrical circuit produce radio waves. Waves themselves may be regarded as vibrations. Water waves, for example, are vibrations set up in water by the wind, by earthquakes, and by objects that drop into or pass through water. Waves may be reflected (bent backward) by various substances, and they may be refracted (bent at an angle) when they pass from one substance to another. Diffraction is the bending of a wave about an obstacle. Interference results when two waves meet.
Waves transmit energy, but not matter. When waves pass through a substance, such as air, the molecules that make up the substance move to and fro but do not progress forward with the wave. The molecules nearest the source of the wave move first. Their movements set adjoining molecules into motion, and in this way the wave travels forward.
In a longitudinal wave, the molecules move to and fro parallel to the direction in which the wave advances. Sound waves are longitudinal waves made up of a series of compressions and rarefactions. In a transverse wave, the molecules move back and forth at right angles to the direction in which the wave advances. A wave on the surface of a body of water is a transverse wave. Electromagnetic waves are also transverse waves. In electromagnetic waves, electric and magnetic fields—rather than molecules—move back and forth.
Three important characteristics of waves are frequency, wavelength, and amplitude. The frequency is the number of waves that pass a given point per unit of time. It is usually expressed in hertz, the number of cycles, or complete vibrations, per second. For example, the frequency of sound waves produced by the lowest note on a piano is 27.5 hertz (27.5 cycles per second), meaning that 27.5 complete waves are formed during each second that the note is sounded.
Wavelength is the distance between two corresponding parts of adjacent waves. In a transverse wave, the wavelength is commonly measured between the crests or troughs of two adjacent waves; in a longitudinal wave, between points of maximum compression or rarefaction. The wavelength multiplied by the frequency gives the speed of a wave, the distance the wave travels per unit of time.
The amplitude of a wave is the greatest amount by which a changing property of a wave varies from its normal value. In a water wave, for example, the amplitude is half the distance between its trough and crest. The more energy a wave has, the greater is its amplitude. The amplitude of a sound wave, for example, is large for a loud sound and small for a soft one.