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# Speed

In free space itâ€™s speed
In medium, , where Âµ0 = absolute permeability and Îµ0 = absolute permittivity.

# Energy

The energy in an EM waves is divided equally between the electric and magnetic fields.

Energy density of electric field, , energy density of magnetic field,

The total energy per unit volume is u = ue + um . Also  .

# Intensity (I)

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

# Momentum

EM waves also carries momentum, if a portion of EM wave of energy u propagating with speed c, then linear momentum .

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 â€“p =2u/c.

# Poynting vector

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
• Because in EM waves  and  are perpendicular to each other, the magnitude of  is .
• The direction of  does not oscillate but its magnitude varies between zero and a maximum (Smax = E0B0/Âµo each quarter of a period.
• Average value of poynting vector is given by

The direction of the poynting vector  at any point gives the waveâ€™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 .

For vacuum or free space,