单词	wave
释义	wave A periodic disturbance in a medium or in space. In a travelling wave (or progressive wave ) energy is transferred from one place to another by the vibrations (see also stationary wave). In a wave passing over the surface of water, for example, the water rises and falls as the wave passes but the particles of water on average do not move forward with the wave. This is called a transverse wave because the disturbances are at right angles to the direction of propagation. The water surface moves up and down while the waves travel across the surface of the water. Electromagnetic waves (see diagram) are also of this kind, with electric and magnetic fields varying in a periodic way at right angles to each other and to the direction of propagation. In sound waves, the air is alternately compressed and rarefied by displacements in the direction of propagation. Such waves are called longitudinal waves . The chief characteristics of a wave are its speed of propagation , its frequency , its wavelength , and its amplitude . The speed of propagation is the distance covered by the wave in unit time. The frequency is the number of complete disturbances (cycles) in unit time, usually expressed in hertz. The wavelength is the distance in metres between successive points of equal phase in a wave. The amplitude is the maximum difference of the disturbed quantity from its mean value. Generally, the amplitude (a) is half the peak-to-peak value. There is a simple relationship between the wavelength (λ) and the frequency (f), i.e. λ = c/f , where c is the speed of propagation. The energy transferred by a progressive sine wave (see diagram) is proportional to a2f2 . See also simple harmonic motion. Electromagnetic waves Sine wave
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