A plane electromagnetic wave of frequency 75 Hz travels in free space along the X-direction. At a particular point in space and time \( \vec{E} = 9.6\,\hat{\jmath} \). What is \( \vec{B} \) at that point? (E = Electric field, B = Magnetic field)
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( 3.2\times10^{-8}\,\hat{k}\,\mathrm{T} \)
In a plane electromagnetic wave the fields are related by \( E = cB \), and \( \vec{E} \), \( \vec{B} \) and the direction of propagation form a right-handed set: \( \vec{E}\times\vec{B} \) points along the direction of travel.
\( \displaystyle B = \frac{E}{c} = \frac{9.6}{3\times10^{8}} = 3.2\times10^{-8}\,\mathrm{T} \)
The wave travels along \( +x \) and \( \vec{E} \) is along \( \hat{\jmath} \). Since
\( \hat{\jmath}\times\hat{k} = \hat{\imath} \)
the magnetic field must be along \( \hat{k} \). So \( \vec{B} = 3.2\times10^{-8}\,\hat{k}\,\mathrm{T} \). The frequency is not needed.
Note: NTA picked this question from an NCERT (2026-27) example and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Nature of Electromagnetic Waves – …
