A plane electromagnetic wave of frequency \( 20 \) MHz travelling in vacuum has its electric field and magnetic field at a specific point and time given as \( \vec{E}_0 = -5\hat{k}\,\mathrm{V/m} \), \( \vec{B}_0 = -16.7\hat{\jmath}\,\mathrm{nT} \). The direction of propagation of the wave is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( -x \) Direction
An electromagnetic wave travels along the direction of \( \vec{E}\times\vec{B} \) (the Poynting vector).
\( \vec{E}_0 = -5\hat{k}\,\mathrm{V/m} \)
\( \vec{B}_0 = -16.7\hat{\jmath}\,\mathrm{nT} \)
\( \vec{E}\times\vec{B} \propto (-\hat{k})\times(-\hat{\jmath}) = \hat{k}\times\hat{\jmath} = -\hat{\imath} \)
So the wave travels in the \( -x \) direction.
The data are consistent: \( E/B = 5/(16.7\times10^{-9}) \approx 3\times10^{8}\,\mathrm{m\,s^{-1}} = c \). The frequency is not needed.