MAHE Manipal B.Sc Nursing 2025
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Energy Density and Intensity of EM waves is considered one the most difficult concept.
48 Questions around this concept.
A radiation of energy 'E' falls normally on a perfectly reflecting surface. The momentum transferred to the surface is (C=Velocity of light)
Out of the following options which one can be used to produce a propagating electromagnetic wave?
Energy Density and Intensity of EM waves-
1. Energy Density-
The electric and magnetic fields in a plane electromagnetic wave are given by
In any small volume
And the energy of the magnetic field is
Thus, the total energy is
The energy density is
For taking average over a long time, the
So,
Now, as we know,
or,
Also
Intensity (I) : The energy crossing per unit area per unit time, perpendicular to the direction of propagation of EM wave is called intensity.
So,
Momentum : Electromagnetic waves also carries momentum. As we know that that the linear momentum is associated with energy and speed, similarly if a portion of EM wave of energy u propagating with speed c, then linear momentum
So we can write that if wave incident on a completely absorbing surface then momentum delivered will be equal to -
But if the wave is incident on totally reflected surface, then the momentum will be equals to -
Poynting vector
Unit of Poynting vector is Watt
Importance of poynting vector is that the direction of the poynting vector
Average value of poynting vector -
As we can notice that direction of poynting vector can be given by vector product so, The direction of
Radiation pressure : It is defined as the momentum imparted per second pre unit area on which the light falls
So, for the perfectly absorbing body, we can write in terms of poynting vector -
And for perfectly reflecting surface -
Wave impedance (Z) : As the word impedance tell that obstruction in flowing of something, similarly here ,the medium offers hindrance to the propagation of wave. Such hindrance is called wave impedance and it is given by -
For vacuum or free space -
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