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In free space it’s speed
In medium, 78584.png, where µ0 = absolute permeability and ε0 = absolute permittivity.


The energy in an EM waves is divided equally between the electric and magnetic fields.
Energy density of electric field, 78590.png, energy density of magnetic field, 78597.png
The total energy per unit volume is u = ue + um 78603.png. Also 78609.png 78618.png.

Intensity (I)

The energy crossing per unit area per unit time, perpendicular to the direction of propagation of EM wave is called intensity.


EM waves also carries momentum, if a portion of EM wave of energy u propagating with speed c, then linear momentum 78638.png.
If wave incident on a completely absorbing surface, then momentum delivered p = u/c. If wave incident on a totally reflecting surface, then momentum delivered p =2u/c.

Poynting vector 78644.png

In EM waves, the rate of flow of energy crossing a unit area is described by the Poynting vector.
  • Its unit is Watt/m2 and 78656.png
  • Because in EM waves 78666.png and 78672.png are perpendicular to each other, the magnitude of 78678.png is 78685.png.
  • The direction of 78691.png does not oscillate but its magnitude varies between zero and a maximum (Smax = E0B0/µo each quarter of a period.
  • Average value of poynting vector is given by
The direction of the poynting vector 78706.png at any point gives the wave’s direction of travel and direction of energy transport the point.

Radiation pressure

Is the momentum imparted per second pre unit area. On which the light falls.
For a perfectly reflecting surface, Pr = 2S/c;
S = poynting vector; c = speed of light.
For a perfectly absorbing surface, Pa = S/c.

Wave impedance (Z)

The medium offers hindrance to the propagation of wave. Such hindrance is called wave impedance and it is given by 78714.png.
For vacuum or free space, 78723.png

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