Given below are two statements:
Statement I: An electromagnetic wave carries energy, and this energy comes at the expense of the energy of the source that produces the wave.
Statement II: A charge oscillating with frequency \( \nu \) radiates electromagnetic waves of frequency \( 2\nu \).
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Statement I is correct but Statement II is incorrect.
Accelerated charges radiate electromagnetic waves. A charge oscillating about a point sets up an oscillating electric field. This field produces an oscillating magnetic field. The two fields keep producing each other as the wave moves out.
Statement I An electromagnetic wave carries energy, and this energy comes at the expense of the energy of the source that produces the wave. — Correct. Radio signals and sunlight both carry energy from one place to another. The energy that the wave takes away is drawn from the accelerated charge that produces it.
Statement II A charge oscillating with frequency \( \nu \) radiates electromagnetic waves of frequency \( 2\nu \). — Incorrect. The fields set up by the charge oscillate at the frequency of the charge itself. The radiated wave therefore has the same frequency \( \nu \) as the oscillating charge.
NCERT (2026-27):
Sources of Electromagnetic Waves – “… It is an important result of Maxwell’s theory that accelerated charges radiate electromagnetic waves. … The frequency of the electromagnetic wave naturally equals the frequency of oscillation of the charge. The energy associated with the propagating wave comes at the expense of the energy of the source – the accelerated charge.”