Energy Density And Intensity Of EM Waves - Practice Questions & MCQ

Energy Density and Intensity of EM waves-

1. Energy Density-

For taking the average over a long time, the sin² terms have an average value of  

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 linear momentum is associated with energy and speed.

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 a totally reflected surface, then the momentum will be equal to - 

Poynting vector ( ) : It is defined as the rate of flow of energy crossing a unit area in electromagnetic waves. So, 

Unit of Poynting vector is Watt/m². Now, as we know that in electromagnetic waves,  and  are perpendicular to each other. So, 

The importance of the Poynting vector is that the direction of the Poynting vector  at any point gives the wave's direction of travel and direction of energy transport to the point.

The average value of the Poynting vector - 

As we can notice that direction of Poynting vector can be given by the vector product so, The direction of does not oscillate but its magnitude varies between zero and a maximum each quarter of the period.

Radiation pressure: It is defined as the momentum imparted per second per unit area on which the light falls

So, for the perfectly absorbing body, we can write in terms of the Poynting vector - 

And for perfectly reflecting surface - 

Wave impedance (Z): As the word, impedance tells that obstruction inflowing of something, similarly here, the medium offers hindrance to the propagation of the wave. Such hindrance is called wave impedance and it is given by - 

For vacuum or free space -