Question

In an electromagnetic wave, the direction of the magnetic induction $\overrightarrow B$ is
A. Parallel to the electric field $\overrightarrow E$
B. Perpendicular to the electric field $\overrightarrow E$
C. Antiparallel to the poynting vector $\overrightarrow S$
D. Random

Answer 1

To select the correct answer, analyze each option one by one. First understand what electromagnetic waves are. Then analyze each option and check which of the given options satisfies the condition for electromagnetic waves to be produced.

Complete step by step answer:
Given, to find the direction of the magnetic induction $\overrightarrow B$ in an electromagnetic wave.First let us know what electromagnetic waves are. Electromagnetic waves are waves produced by oscillating charges.Let us analyze each option one by one.

Parallel to the electric field $\overrightarrow E$ :- When magnetic induction or magnetic field is parallel to electric field, then the magnetic field will not have any effect on the charged particle and it will move along the direction of electric field. That is the wave is not an electromagnetic wave as the charge would not oscillate. Thus, option A is incorrect.

Perpendicular to the electric field $\overrightarrow E$ :- When the magnetic field is perpendicular to the electric field, the path of the charged particle will be circular or we can say the charge particle will oscillate and we know oscillating charges produce electromagnetic waves. Thus, option B is correct.

Antiparallel to the poynting vector $\overrightarrow S$:- The poynting vector is given by the formula, $\overrightarrow S = \dfrac{1}{{{\mu _0}}}\left( {\overrightarrow E \times \overrightarrow B } \right)$ that means the poynting vector is perpendicular to the magnetic field. Thus, option C is incorrect.

Random :- If we consider random direction we can’t say how a charged particle will be affected by the magnetic field. Thus, option D is incorrect.

Hence, the correct option is option B.

Note: There are seven types of electromagnetic waves, which are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. These seven types of electromagnetic waves are known as electromagnetic spectrum. Also remember, electromagnetic waves travel with the speed of light.