Question

The maximum frequency of transmitted radio waves above which the radio waves are no longer reflected back by ionosphere is ……………….(N= maximum electron density of ionosphere, $g$=acceleration due to gravity):
a. $gN$
b. $g{N^2}$
c. $g\sqrt N $
d. ${g^2}{N^2}$

Answer 1

Hint: The maximum electron density is defined as the electrons per cubic meter in the ionosphere. The ionosphere is an ionized part of the atmosphere that gets ionized due to solar radiation. The ionosphere consists of free electrons which help in reflecting the radio waves.


Complete step by step answer:
The frequency at which or below which a radio wave will get reflected by the ionosphere and above which it will get penetrate through ionosphere is called as the critical frequency and this critical frequency is given by,
$ \Rightarrow {f_c} \simeq g\sqrt N $
Where acceleration due to gravity is g and the maximum electron density is given by N.

Hence, the correct answer is option (C).

Additional information:
Ionosphere is the layer of the atmosphere of the earth which ranges from 48 km to 965 km approx., the ionization of the ionosphere is done by solar radiation. Most of the communication signals are reflected by the ionosphere but the high frequency signal gets past the ionosphere. There are many layers present in the ionosphere namely E, D and F layer, the layers have different levels of ionization. The ionosphere also has some parts of the thermosphere and exosphere. The high frequency band works best under critical frequency works best and during night the lower frequency works best because the radiation during the day and the night are different.

Note: The frequency at which the radio waves penetrate the ionosphere is known as critical frequency and it is represented by ${f_c}$ and the range of the maximum electron density in the ionosphere is ${10^{10}}$ to ${10^{13}}$ electrons per cubic meter.