Question

The direction of propagation of electromagnetic wave is along
(A). Electric field vector, $\overrightarrow{E}$
(B). Magnetic field vector, $\overrightarrow{B}$
(C). $\overrightarrow{E}\cdot \overrightarrow{B}$
(D). $\overrightarrow{B}\times \overrightarrow{E}$

Answer 1

Hint: Electromagnetic waves are made of oscillating electric and magnetic fields. Unlike mechanical waves, they do not require a medium to travel from one place to the other. EM waves have magnitude as well as direction, so they can be represented by a vector in space.

Complete step-by-step solution:
When an electric field interacts with a magnetic field, electromagnetic waves are generated. Electromagnetic waves are vibrations which occur between an electric field and a magnetic field.

Electric field is defined as the work done to bring a unit positive charge from infinity to a point in the field. While magnetic field is defined as the work done to bring a north pole from infinity to a point in the field.

The electromagnetic waves do not require a medium to travel; it can even travel in a vacuum. Light is an example of electromagnetic waves.

The electric field and magnetic field vibrate perpendicular to each other. The resultant electromagnetic waves are perpendicular to the direction of electric field and magnetic field. Therefore, the magnetic field, electric field and the electromagnetic waves oscillate perpendicular to each other.

Therefore, the electromagnetic waves are given by-
$\overrightarrow{M}=\overrightarrow{B}\times \overrightarrow{E}$
Here,
$\overrightarrow{M}$ is the vector representing the electromagnetic waves
$\overrightarrow{B}$ is the magnetic field vector
$\overrightarrow{E}$ is the electric field vector

Therefore, the electromagnetic waves are vector product of electric and magnetic field and its direction is perpendicular to them,
$\overrightarrow{M}=\overrightarrow{B}\times \overrightarrow{E}$

Hence, the correct option is (D).

Note:
The velocity of EM waves is constant in a vacuum and is equal to $3\times {{10}^{8}}m{{s}^{-1}}$ which is also the speed of light in vacuum. The highest point of the wave is called the crest while the lowest point is called the trough. EM waves show all the properties of waves like interference, diffraction etc.