Question

The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic wave is:

• A. $c:1$
• B. $c^2:1$
• C. $1:1$
• D. $\sqrt{c}:1$

Answer 1

Answer: (C)

The intensity of an electromagnetic wave is given by:

$I=U_{av}c$

here, $U_{av}$ is the average energy and $c$ is the speed of light.

The average energy in terms of the electric field is written as:

$U_{av}=\frac{1}{2}{\epsilon }_o{E}_o^2$

Average energy in terms of magnetic field:

$U_{av}=\frac{1}{2}\frac{B_o^2}{{\mu }_o}$

We know that, $E_o=c{B}_o$ and $c=\frac{1}{\sqrt{{\mu }_o{\epsilon }_o}}$

$\therefore U_{av\left(electricfield\right)}=\frac{1}{2}{\epsilon }_o\times \frac{1}{{\mu }_o{\epsilon }_o}{B}_o^2$

$\Rightarrow \frac{1}{2}\frac{B_o^2}{{\mu }_o}$

Since the energy due to electric and magnetic field is same. so, both will have the same contribution to the intensity.