A 50m long antenna is mounted on a 200 m tall building. The complex can become transmission tower for waves with wave length
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( \sim 200\,\mathrm{m} \)
An antenna radiates a signal efficiently when its length is comparable to the wavelength; for a standard antenna the length is about a quarter of the wavelength it transmits.
\( \displaystyle l = \frac{\lambda}{4} \;\Rightarrow\; \lambda = 4l \)
\( \displaystyle \lambda = 4 \times 50\,\mathrm{m} = 200\,\mathrm{m} \)
The height of the building only raises the antenna; it does not change the wavelength the antenna can radiate.
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.”