Which of the following statements about electromagnetic waves is NOT correct?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → They travel with the same speed in all media.
The speed of an electromagnetic wave in a medium is \( v=1/\sqrt{\mu\varepsilon} \). It depends on the permittivity and permeability of the medium. Only in vacuum is it \( c=1/\sqrt{\mu_0\varepsilon_0} \).
Option (1) They are transverse. — Correct statement. The electric and magnetic fields are perpendicular to each other and to the direction of travel.
Option (2) They travel with the same speed in all media. — Incorrect statement. Light moves more slowly in glass or water than in air. The speed changes from one medium to another.
Option (3) They travel in vacuum with the speed of light. — Correct statement. In vacuum all electromagnetic waves travel with \( c \), the speed of light.
Option (4) They are produced by accelerated charges. — Correct statement. A charge that is accelerating gives out electromagnetic waves.
NCERT (2026-27):
Nature of electromagnetic waves – “Thus, the velocity of light depends on electric and magnetic properties of the medium.”