From the following diagram, determine the direction of propagation of an electromagnetic wave.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Inward to the plane of paper
An electromagnetic wave is transverse: \( \vec{E} \), \( \vec{B} \) and the direction of propagation are mutually perpendicular, and the wave travels along \( \vec{E}\times\vec{B} \).
Take \( x \) to the right, \( y \) upward and \( z \) out of the page. Then \( \vec{E} \) is along \( \hat{y} \) and \( \vec{B} \) along \( \hat{x} \).
\( \displaystyle \hat{y}\times\hat{x} = -\hat{z} \)
The direction of propagation is \( -\hat{z} \), which is into the plane of the paper.
NCERT (2026-27):
Nature of Electromagnetic Waves – “It can be shown from Maxwell’s equations that electric and magnetic fields in an electromagnetic wave are perpendicular to each other, and to the direction of propagation.”
Nature of Electromagnetic Waves – “… To find the direction, we note that E is along y-direction and the wave propagates along x-axis. Therefore, B should be in a direction perpendicular to both x- and y-axes. Using vector algebra, E × B should be along x-direction.”