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Wave
Motion, in physics, mechanism by which energy
is conveyed from one place to another in mechanically propagated waves
without the transference of matter. At any point along the path of
transmission a periodic displacement, or oscillation,
occurs about a neutral position. The oscillation may be of air molecules,
as in the case of sound
traveling through the atmosphere; of water molecules, as in waves
occurring on the surface of the ocean; or of portions of a rope or a wire
spring. In each of these cases the particles of matter oscillate about
their own equilibrium position and only the energy moves continuously in
one direction. Such waves are called mechanical because the energy is
transmitted through a material medium, without a mass movement of the
medium itself. The only form of wave motion that requires no material
medium for transmission is the electromagnetic wave; in this case the
displacement is of electric and magnetic fields of force in space .
II. 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 (see Fig. 1). The longitudinal 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. Another type of
wave is the transverse wave, in which the vibrations are at right angles
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 (see Fig. 2); or it may be electromagnetic, such as light,
X ray, or radio waves (see X
Ray). 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.
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.
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