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Types of Waves

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The notation of a wave is something familiar to everyone in one form or another, whether it be ocean waves or sound waves. In many cases an observed wave is the result of a disturbance moving through a medium such as water, air or a crowd of people. As the disturbance is transferred from one part of the medium to another, we are able to observe the location of the disturbance as it moves with speed in a particular direction. Any quantitative measurement or feature of the medium which clearly identifies the location and velocity of the disturbance is called a signal.The signal may distort; however, as long as it remains recognizable, it can be used to identify the motion of the disturbance. Different types of waves are described in the below section.

Mechanical waves

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A mechanical wave is a kind of wave or disturbance which propagates through some substance called the medium for the wave. For a mechanical wave a medium is compulsory for propagation. As the wave travels through the medium, the particles that make up the medium undergo displacements of various kinds, depending on the nature of the wave. Transverse and longitudinal waves are the examples of mechanical waves.
On the basis of direction of vibration of the particles during the transmission of progressive wave in a material medium, the wave motion or mechanical waves are divided in to two categories: i) transverse waves and ii) longitudinal waves.

A wave in which each particle of the medium vibrates about its mean position of rest in a direction at right angles to the direction of propagation of a wave is called a transverse wave. The examples of transverse waves are the waves on the surface of the water, the vibrations of a string, electromagnetic waves, light waves, heat waves, X-rays etc.

Transverse waves
In a transverse wave, if the displacement of a particle at any instant is maximum in position direction, it is said to be at the crest of the wave at that instant. On the other hand, if a particle at any instant has a negative maximum displacement, it is called as a trough of the wave at that instant. The transverse waves can easily be demonstrated by setting up waves along a string or cord stretched horizontally by moving it slightly up an down and up again. When a string is tied at one end and moved up and down through the other end then a disturbance or a wave is propagated along the length of the string, that is, the particles of the string oscillate along a direction right angles to the direction along which the wave propagated. Hence the waves formed in the string are transverse.
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A wave in which the particles of the medium move to and fro about their mean position of rest along the direction in which the wave propagates through the medium is called longitudinal wave. In a longitudinal wave each oscillating particle lags in phase by a constant amount behind its predecessor. In this type of wave motion the particles o f the medium are alternately crowded together and spread out. The place where the particles are crowded together is called a region of compression or condensation. On the other hand, the place where the particles are farther apart than their normal separation, is known as a region of rarefaction.

Longitudinal waves
Sound waves traveling in the air; the waves produced in the interior of liquids; waves produced in air-columns inside pipes; waves set up in the rod; waves produced in a vertical helical spring by pulling its lower loaded end up and down and up again etc. are the some of the examples of longitudinal waves.
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Surface waves

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Surface wave is described as a mechanical wave which propagate through the interface between two media especially in the case of liquids. This liquids have different density, hence we can differentiate separately. It can also be an electromagnetic wave. In the case of radio transmission, ground wave is act as a surface wave.
Electromagnetic waves are formed due to the combined effect of electric and magnetic fields. These two are perpendicular to each other. Electromagnetic waves can propagate even in the absence of a medium. Any wave can transport their energy while propagation. Electromagnetic wave can travel at the speed of 3$\times10^{8}$ m/s in vacuum. Light, X-rays, micro waves, radio waves etc are some of the examples of electromagnetic waves. These waves produce the radiations.

Electromagnetic waves
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Matter waves

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The concept of matter waves is possible to explain how the electrons could be stable in their orbits around the nucleus. If an integral number of electron wavelengths fitted into the circumference of the electron orbit, standing waves would be possible and no energy would be lost. According to De-Broglie hypothesis, the particle of mass m and velocity v is associated with a wave. This wave is called matter wave. The wavelength of matter wave is
$\lambda$ = $\frac{h}{mv}$
Matter waves are not electromagnetic wave since they do not depend on the charge of the particle. The velocity of the matter waves is not constant. But it depends on the velocity of the particle. Light wave has got same velocity, for all wavelengths. But in the case of matter waves, the velocity is inversely proportional to the wavelength. Matter waves can be represented symbolically.

Standing Waves

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Sometimes a wave motion appears to stay in one position or to stand still; when this happens we call it a standing or stationary wave. Standing waves are formed in musical instruments when resonance occurs. Standing waves produced by interference as a result of the superposition of two waves when a traveling wave is reflected back along its incident path. Standing waves occur by resonance only at the natural frequencies of vibration of a string or medium. Standing waves have nodes where there is no displacement at any time. Nodes are formed by destructive interference as the wave traveling to the right tries to displace the string upwards and the reflected wave traveling to the left tries to displace it downwards. Only standing waves have nodes.