Question

The propagation of radio waves with frequency $2MHz$ to $30MHz$ is due to
A) Space wave
B) optical wave
C) Ground wave
D) Sky wave

Answer 1

Hint: Radio waves propagate with different frequencies in different forms. The given frequency range falls into the category of short/medium frequency waves. These short/medium frequency waves propagate with the help of the ionosphere, which is a charged layer of the atmosphere. We proceed by scrutinizing all the given options to arrive at the correct answer.

Complete answer:
Firstly, let us go through the given options one by one, to match the required answer to the given question. Radio waves propagate with different frequencies in different forms, as follows:
Ground wave propagation: Ground wave propagation is used for low-frequency range transmissions, mostly less than $1MHz$. This type of propagation uses the ground/surface region of the Earth to transmit radio waves. Transmission of signals over large distances is usually not carried out using this method since this causes severe attenuation, which increases with increasing frequency of the waves.
Sky wave propagation: Sky wave propagation is used for the propagation of EM waves with a frequency range of $3MHz-30MHz$. This type of propagation makes use of the ionosphere, which is a charged layer of atmosphere, away from the surface of the Earth. The charges/ions present in this layer provide a reflecting medium to the radio or communication waves, within a particular frequency range. The waves are either reflected or refracted back to the surface of the Earth. This property of the ionosphere is used for long-distance transmission of the waves without much attenuation and loss of signal strength. Both medium and shortwave frequencies can be reflected/refracted back to the Earth, which makes this type of propagation of radio waves useful in transcontinental transmissions.
Space wave propagation: Space wave propagation is used for propagation of EM waves with a frequency range of $30MHz-300MHz$. Space satellite communication as well as very high-frequency waves use this type of propagation method. This type of propagation makes use of the troposphere, in order to carry out the propagation. It basically involves sending a signal of particular strength in a line from the transmitter to the receiver. Here, we must ensure that for very large distances, the height of the tower used for transmissions is high enough to prevent waves from touching the earth’s curvature, thus preventing attenuation and loss of signal strength.
Optical wave propagation: Optical wave propagation is a special category of radio wave propagation, which allows propagation in free space, guided with the help of dielectric waveguides. Optical fibers are manufactured to enable such dielectric waveguides, for this type of radio wave propagation.

From the above explanation, we can conclude that the correct answer is option D.

Note:
In sky wave propagation, it is important to ensure that radio waves are transmitted at particular angles of emission, from the ground. The transmitters need to emit these EM waves at a critical angle, to ensure total reflection to the ground, just like the total internal reflection of optic waves. If not, these waves may escape into space.