2021 Electromagnetic WavesElectromagnetic Waves – Directions of E, B and Propagation Medium Question 19 of 49 in Electromagnetic Waves

For a plane electromagnetic wave propagating in x-direction, which one of the following combination gives the correct possible directions for electric field ($E$) and magnetic field ($B$) respectively?

A $-\hat{j}+\hat{k},\ -\hat{j}-\hat{k}$
B $\hat{j}+\hat{k},\ -\hat{j}-\hat{k}$
C $-\hat{j}+\hat{k},\ -\hat{j}+\hat{k}$
D $\hat{j}+\hat{k},\ \hat{j}+\hat{k}$

The answer is A.

Solution

For an EM wave, $\vec{E}$, $\vec{B}$ and the direction of propagation are mutually perpendicular, and $\vec{E} \times \vec{B}$ is along the direction of propagation ($\hat{i}$). For $\vec{E} = -\hat{j} + \hat{k}$ and $\vec{B} = -\hat{j} - \hat{k}$: $\vec{E} \times \vec{B} = 2\hat{i}$ (parallel to propagation) and $\vec{E} \perp \vec{B}$. In the other combinations $\vec{E} \times \vec{B} = 0$ (E and B parallel), so they are not possible.

Concept E × B along direction of propagation

Electromagnetic Waves in past papers

49 questions from this chapter have appeared across 24 exam years.