Identify the correct feature of the electromagnetic waves among the following.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The direction of propagation of an electromagnetic wave is perpendicular to both electric and magnetic fields associated with it.
Electromagnetic waves are transverse: \( \vec{E} \), \( \vec{B} \) and the direction of propagation are mutually perpendicular, and the waves need no material medium.
Option (1) The direction of propagation of an electromagnetic wave is perpendicular to both electric and magnetic fields associated with it. — Correct. The wave travels along \( \vec{E}\times\vec{B} \), which is perpendicular to both fields.
Option (2) The velocity of electromagnetic waves is medium independent. — Incorrect. The speed \( v = 1/\sqrt{\mu\varepsilon} = c/n \) depends on the medium.
Option (3) The material medium is involved in the vibrations of the electric and magnetic fields. — Incorrect. The electric and magnetic fields themselves oscillate; no material particles vibrate, which is why the waves travel through vacuum.
Option (4) When the electromagnetic waves enter from a rarer medium to a denser medium, their wavelength and frequency both decrease. — Incorrect. The frequency does not change on refraction; only the speed and the wavelength decrease in the denser medium.
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 – “… They differ from all the other waves we have studied so far, in respect that no material medium is involved in the vibrations of the electric and magnetic fields.”
Nature of Electromagnetic Waves – “… Thus, the velocity of light depends on electric and magnetic properties of the medium.”