A 100 m long antenna is mounted on a 25 m tall building. The complex can become a transmission tower for waves with wavelength nearly equal to:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 400 m
For efficient radiation, the size of an antenna must be comparable to the wavelength; a standard antenna has a length of about \( \lambda/4 \). The height of the building affects how far the signal reaches, not the wavelength the antenna is suited to.
\( \displaystyle l = \frac{\lambda}{4} \;\Rightarrow\; \lambda = 4l = 4 \times 100\,\mathrm{m} = 400\,\mathrm{m} \)
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Size of the Antenna or Aerial – “For transmitting a signal, we need an antenna or an aerial. This antenna should have a size comparable to the wavelength of the signal (at least λ/4 in dimension) so that the antenna properly senses the time variation of the signal.”