Previous Year Question
A layer in the Earth’s atmosphere called ionosphere facilitates radio communication. Why?
The presence of ozone causes the reflection of radio waves to Earth.
Radio waves have a very long wave length.
Which of the statements given above is/are correct?
Correct Answer: D
Detailed Solution
Statement 1 is incorrect: Ozone does not return radio waves towards the Earth.
The ozone layer is located mainly in the stratosphere.
Its principal function is to absorb harmful ultraviolet radiation from the Sun.
In the ionosphere, free electrons and ions produced by solar ultraviolet radiation and X-rays affect the propagation of radio waves.
Statement 2 is also incorrect: Radio waves have comparatively long wavelengths within the electromagnetic spectrum, but a long wavelength alone does not explain ionospheric radio communication.
The actual process is:
Electron density in the ionosphere varies with altitude, time of day, season and solar activity.
Radio waves of certain frequencies, particularly the HF band of approximately 3–30 MHz, may be progressively refracted by ionised layers and returned towards the Earth.
This allows radio signals to travel beyond the curvature of the Earth and far beyond the visible horizon.
Not all radio waves return from the ionosphere. Very high-frequency waves may pass through it into space, while some lower frequencies may be absorbed.
Thus, ionospheric radio communication is enabled by the interaction of particular radio frequencies with free electrons, a reason not stated in either statement.