Among the following which one is wrong for sky wave propagation?

The correct answer is Option (4) → Sky waves are radio waves of very high frequency 30 MHz to 300 MHz

In sky wave propagation, radio waves sent upward are reflected back to the earth by the ionosphere. The ionosphere reflects only waves of frequency from a few MHz up to about 30 MHz; waves of higher frequency pass through it and escape into space.

Option (1) Sky waves are reflected back to the earth by ionosphere — True for sky waves. Reflection by the ionosphere is what makes sky wave propagation possible.

Option (2) Sky waves are radio waves of frequency range 2 MHz to 30 MHz — True for sky waves. This is the band the ionosphere can reflect.

Option (3) Sky wave propagation is used by short wave broadcast services — True for sky waves. Short wave broadcast services use this band and reach long distances by sky waves.

Option (4) Sky waves are radio waves of very high frequency 30 MHz to 300 MHz — Wrong statement. Waves of 30 MHz to 300 MHz (VHF) penetrate the ionosphere and are not reflected back, so they travel as space waves, not sky waves.

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):
Sky Waves – “In the frequency range from a few MHz up to 30 to 40 MHz, long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth. This mode of propagation is called sky wave propagation and is used by short wave broadcast services.”

Sky Waves – “… The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape.”