Question

Which of the following is not the characteristics of electromagnetic wave?

• A. The electric and magnetic fields $\overrightarrow{E}$ and $\overrightarrow{B}$ are always perpendicular to the direction in which the wave is travelling
• B. The electric field is always perpendicular to the magnetic field
• C. The cross product $\overrightarrow{E}\times \overrightarrow{B}$ always gives the direction in which the wave travels
• D. The fields do not vary simultaneously

Answer 1

Answer: (D)

The fields do not vary simultaneously

Maxwell predicted that an accelerated charge produces a sinusoidal time varying magnetic field, which in turn produces a sinusoidal time varying electric field. These two fields are mutually perpendicular to each other and sources of each other. They (fields) form an electromagnetic wave i.e. the wave that is propagated by simultaneous periodic variations of electric field ($\overrightarrow{E}$)and magnetic field intensity ($\overrightarrow{B}$).

The wave so produced, propagates in the direction of $\overrightarrow{E}\times \overrightarrow{B}$, this direction is perpendicular to both $\overrightarrow{E}$ and $\overrightarrow{B}$, by right hand screw rule or $\overrightarrow{E}\times \overrightarrow{B}$ gives the direction in which wave travels.