# Speed

In free space itâ€™s speed

In medium, , where

*Âµ*_{0}= absolute permeability and*Îµ*_{0}= 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*=*u*+_{e}*u*. Also ._{m}# 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*=2*u*/*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/m
^{2}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 (
*S*_{max}=*E*_{0}*B*_{0}/*Âµ*o 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,

*P*= 2_{r}*S*/*c*;*S*= poynting vector;

*c =*speed of light.

For a perfectly absorbing surface,

*P*=_{a}*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,