## waveFamiliar examples of waves are the surface waves on water, or transverse waves on a stretched rope; sound is carried through air by a wave motion in which the air molecules vibrate parallel to the direction of propagation, and light and other forms of electromagnetic radiation involve electromagnetic fields oscillating perpendicular to it. Waves have high points called crests and low points called troughs. The vibration between two extreme points – the crest and the trough – is generally an "oscillation." Amplitude refers to the maximum distance on either side of the midpoint of the wave. The distance from the top of one crest or tough to the next is called the wavelength. How frequently a point on a wave passes a particular point per unit time is described by its frequency. The unit of frequency is the hertz (Hz) after Heinrich Hertz. Once per second is 1 hertz, twice per second is 2 hertz, etc. The product of the wavelength and the frequency gives the velocity of propagation of the wave. The time it takes for an object to make a complete vibration – for instance, the time it takes a pendulum to swing once each way – is called its period. Frequency and period are reciprocals of each other. According to the direction and form of the local oscillations of the medium, different polarizations of the wave are distinguished. Standing waves (also called stationary waves), where the nodes and antinodes – points of zero and maximum amplitude – appear not to move) arise where identical waves traveling in opposite directions superpose. The characteristic properties of waves include propagation in straight lines; reflection at plane surfaces; refraction – a change in direction of a wave transmitted across a plane interface between two media; diffraction – diffuse scattering by impenetrable objects of a size comparable with the wavelength; and interference – the cancellation of one wave by another wave half a wavelength out of step (or phase) so that the crests of one wave fall on the troughs of the other. If the wave velocity is the same for all wavelengths, then quite arbitrary forms of disturbance will travel as waves, and not simply regular successions of crests and troughs. When this is not the case, the wave is said to be dispersive and localized disturbances move at a speed (the group velocity) quite different from that of the individual crests, which can be seen moving faster or slower within the disturbance "envelope," which becomes progressively broader as it moves. Waves carry energy and momentum with them just like solid objects; the identity of the apparently irreconcilable wave and particles aspects of matter is the basic tenet of quantum mechanics. ## Related category• WAVES AND WAVE PHENOMENASource: U.S. Centennial of Flight Commission | ||||||

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