3
$\begingroup$

The Rayleigh scattering effect applies to 'light' signals, and the scattering of a signal when passing through a material medium, the amount of light scattered is proportional to 1/$\lambda^4$. I am curious as to why(or if) this wasn't considered when we established global communication systems through the various means of modulation. The high-frequency carrier/modulated signal used in say, FM, should visibly have a small wavelength, and since it is an EM wave too(like visible light or others) shouldn't we be concerned about its scattering as much as we are concerned about the consumed bandwidth or SNR etc. ?

I am not sure if I should be posting this in the Stack PE, I request the moderators to kindly direct me. Tag/post edits are welcome. Thank you!

$\endgroup$
2
  • 4
    $\begingroup$ optical signals are in the range of multiples of 100 THz in frequency. In contrast, microwave signals in communications are <100 GHz to my knowledge. That is at least three orders of magnitude difference in frequency, so the Rayleigh scattering cross section will be 12 orders of magnitude lower. $\endgroup$
    – wcc
    Sep 17 '18 at 3:29
  • 3
    $\begingroup$ I happened to work with communication channels that were modeled with Rayleigh scattering. There, the objects that scattered radio signal were not air molecules, but were buildings, vehicles, and other objects. It is a big concern in vehicle communication or in cell phone communication. Many ways were invented to fight against this problem. $\endgroup$
    – verdelite
    Sep 17 '18 at 4:23
4
$\begingroup$

Rayleigh scattering affects light passing through air, and the light wavelengths range from ~650 nm (red) to ~420 nm (blue) compared to the sizes of the gas molecules in air of ~$4 \unicode{x212B}$ (0.4 nm). That is, the air molecules are about 1/1000 of the wavelength of blue light, which is much strongly scattered than red light.

As red light is quite weakly scattered, and has a wavelength about $1.5 \times$ that of blue light, radio communications signals with wavelengths ranging from metres down to centimetres are barely going to be scattered since those wavelengths are $100,000 \times$ (or more) the wavelength of blue light.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.