Accelerated charges radiate electromagnetic waves. Choose the correct key features of electromagnetic waves from the following:
(A) The electric field is always parallel to the magnetic field.
(B) The electric and magnetic fields are always normal to the direction in which the wave is travelling.
(C) The electric and magnetic fields sometimes vary sinusoidally.
(D) The electric and magnetic fields vary with the same frequency.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (D) only
In an electromagnetic wave produced by an oscillating charge, \( \vec{E} \), \( \vec{B} \) and the direction of travel are mutually perpendicular, and the two fields oscillate in phase, with the frequency of the source.
\( E_x = E_0 \sin(kz - \omega t) \)
\( B_y = B_0 \sin(kz - \omega t) \)
(A) The electric field is always parallel to the magnetic field. — Incorrect. \( \vec{E} \) is always perpendicular to \( \vec{B} \).
(B) The electric and magnetic fields are always normal to the direction in which the wave is travelling. — Correct. Electromagnetic waves are transverse; the direction of travel is along \( \vec{E}\times\vec{B} \).
(C) The electric and magnetic fields sometimes vary sinusoidally. — Incorrect. For the wave from a charge oscillating at a fixed frequency, both fields vary sinusoidally at every point and at all times, as in the equations above; the word "sometimes" makes this wrong as a key feature.
(D) The electric and magnetic fields vary with the same frequency. — Correct. Both fields oscillate with the frequency of the accelerated charge, and in phase.
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 – “… The electric field Ex is along the x-axis, and varies sinusoidally with z, at a given time. The magnetic field By is along the y-axis, and again varies sinusoidally with z.”
Sources of Electromagnetic Waves – “… The frequency of the electromagnetic wave naturally equals the frequency of oscillation of the charge.”