Which of the following statements are correct in the context of electromagnetic waves?
(A) The energy associated with a propagating electromagnetic wave comes at the expense of the energy of the source.
(B) We can set up an ac circuit in which the current can oscillate at the frequency of yellow light (about \( 6\times10^{14} \) Hz).
(C) Jagdish Chandra Bose produced and observed electromagnetic waves of wavelength \( 25 \) mm to \( 5 \) mm.
(D) Hertz experimentally demonstrated the existence of electromagnetic waves for all the defined regions of the electromagnetic spectrum.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (C) only
(A) The energy associated with a propagating electromagnetic wave comes at the expense of the energy of the source. — Correct. An electromagnetic wave carries energy, and by conservation of energy this energy is supplied by the radiating source.
(B) We can set up an ac circuit in which the current can oscillate at the frequency of yellow light (about \( 6\times10^{14} \) Hz). — Incorrect. Even for very small L and C, a circuit cannot oscillate at such a frequency; light of this frequency is emitted by atomic transitions, not by circuits.
(C) Jagdish Chandra Bose produced and observed electromagnetic waves of wavelength \( 25 \) mm to \( 5 \) mm. — Correct. Bose worked with these millimetre waves in Kolkata a few years after Hertz.
(D) Hertz experimentally demonstrated the existence of electromagnetic waves for all the defined regions of the electromagnetic spectrum. — Incorrect. Hertz produced and detected only waves of wavelength of the order of metres (radio waves); he did not cover the other regions of the spectrum.
NCERT (2026-27):
Sources of 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.”
Sources of Electromagnetic Waves – “… We might think that all we needed to do was to set up an ac circuit in which the current oscillate at the frequency of visible light, say, yellow light. But, alas, that is not possible. The frequency of yellow light is about 6 × 1014 Hz, while the frequency that we get even with modern electronic circuits is hardly about 1011 Hz. This is why the experimental demonstration of electromagnetic wave had to come in the low frequency region (the radio wave region), as in the Hertz’s experiment (1887).”
Sources of Electromagnetic Waves – “… Seven years after Hertz, Jagdish Chandra Bose, working at Calcutta (now Kolkata), succeeded in producing and observing electromagnetic waves of much shorter wavelength (25 mm to 5 mm).”