Waves
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What are waves?
Waves can be simply defined as a condition that repeats itself with time and space. Electromagnetic waves, mechanical waves, heat waves, matter waves, fear waves etc all all examples of waves. It should be noted here that in the case of propagation of waves there is no need of a transfer of a medium.
TYPES OF WAVES ACCORDING TO THE BASIC REQUIREMENT OF THE MEDIUM
1. MECHANICAL WAVES
These waves require a medium for their propagation. These are thus a disturbance that propagates from one point to the other without the transfer of the medium.
when these waves propagate
-Energy transfer takes place
-linear momentum transfer takes place
-Angular momentum transfer may take place
2. ELECTROMAGNETIC WAVES
These waves propagate in the form of electrical and magnetic vectors. These vectors are perpendicular to each other and also to the direction of motion.
the relationship between them is given by
|
E=V*B |
Thus we see that for the propagation of an electromagnetic wave, ie for the velocity of propagation to be high, the electric field intensity in the medium in which the waves are traveling must be high, whereas the magnetic field and the magnetic field induction must be low. There is no requirement of a medium in the case of electromagnetic waves, these waves can travel in vacuum also. For E to be high the dielectric constant of the medium must be low and for the magnetic vector to be low , the relative magnetic permeability must be low. According to the electromagnetic wave theory the velocity of the vaves is given by
v=1/(mrer)1/2(moeo)1/2
v=c/(mrer)1/2
c/v=(mrer)1/2
n=(mrer)1/2
where n is the refractive index of the material.
since in most of the materials of day to day interest have mr=1, either they are paramagnetic m>1 or diamagnetic mr<1, the refractive index of the material is given by
n=(mr)1/2
Thus we see that the optical properties of the medium are dependant on its electrical properties. Now since the relative permittivity of a medium depends upon the frequency of the waves as
|
er=1+[(enoke2/meo)/(w2ok-w2)] |
where nok is the concentration of electrons of the kth group, wok is the natural frequency of the electrons of the kth group, e is the charge on an electron, m is the mass of an electron, w is the frequency.
Thus for higher frequency the dielectric constant must be high.
CLASSIFICATION OF WAVES ON THE BASIS OF VIBRATION
On the basis of vibration the waves can be classified as
1. TRANSVERSE WAVES
these waves have vibrations perpendicular to the direction of propagation. All electromagnetic waves are transverse in nature. Transverse waves can be polarized.
2. LONGITUDINAL WAVES
longitudinal waves have the direction of propagation along the direction of vibrations. Sound waves are longitudinal in nature. Mechanical waves produced in a gaseous medium are longitudinal. The waves produced in a liquid medium and solid medium are also longitudinal. Longitudinal waves cannot be polarized.
