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## QuestionPhysicsClass 12

Which of the following frequencies will be suitable for beyond-the horizon communication using sky waves?（ ）
A.  $$10\;\mathit{KHz}$$
B. $$10\;\mathit{MHz}$$
C. $$1\mathit{GHz}$$
D. $$1000\;\mathit{GHz}$$

B
4.6
4.6

## Solution

For sky wave,  frequency,$$f=10\times 10^6\mathit{Hz}$$
Wavelength,$$\lambda =\dfrac c f=\dfrac{3\times 10^8}{10^7}=30\;m$$
It gets reflected easily from the ionosphere.
For option $$A$$,  antenna size is not possible.
The high-energy signal waves $$(1\ GHz − 1,000\ GHz)$$ penetrate the ionosphere and escape. Options $$C$$ and $$D$$ are incorrect.
$$10\ MHz$$ frequencies get reflected easily from the ionosphere. Hence, signal waves of such frequencies are suitable for beyond-the-horizon communication.
Hence, option $$(B)$$ is correct.