Summary
Rayleigh Scattering
Rayleigh scattering is the scattering of light or other electromagnetic radiation by particles in the Earth’s atmosphere that are much smaller than the wavelength of the light. These small particles less than 1/10th of a wavelength, such as nitrogen and oxygen molecules, are present even in “clear” air, where more noticeable sources of interference like fog or dust are absent. Particles larger than the wavelength of light give rise to Mie scattering.
As light passes through these particles, it induces oscillations in the particles’ electric fields, causing them to re-emit light in various directions. The amount of scattering depends on the wavelength of the light. Shorter wavelengths (such as blue in the visible spectrum) are scattered much more strongly than longer wavelengths (such as red). This effect explains why the sky appears blue during the day and why sunsets are red when sunlight passes through more of the atmosphere.
In Free Space Optical (FSO) communication systems, Rayleigh scattering contributes to signal attenuation, although it is less significant than other factors like absorption and Mie scattering (caused by larger particles such as aerosols). To reduce scattering effects, FSO systems often use infrared wavelengths, such as 850 nm or 1550 nm, which are less affected by Rayleigh scattering compared to visible light. For example, the scattering of a wavelength at 430nm or blue light is scattered 6 times as efficiently as light at 680nm or red light [2]. By comparison other types of scattering such as Mie scattering are less wavelength dependent.
These scattering particles can be thought of like tiny prisms in the air. The more particles there are, the more the light is scattered, and the wavelength of the light determines how strongly it interacts with these particles.
Mie Scattering
Mie Scattering or Debye scattering is similar to Rayleigh Scattering but caused by particles that are larger than the light’s wavelength, such as particles such as aerosols or raindrops. The direction of the scattering is not uniform – instead the scattering tends to continue in its general original direction. It is also less wavelength dependent, making it more difficult to mitigate with wavelength selection in FSO.
This type of scattering creates the whitish glare around the sun when particles are present, and the white light from mist and fog.
Other Types of Scattering
While Rayleigh and Mie scattering are the most significant for atmospheric phenomena and applications like Free Space Optics (FSO), other types of scattering exist:
Raman Scattering
A weak inelastic scattering process used in spectroscopy but generally negligible in atmospheric systems.
Brillouin Scattering
Relevant for interactions in dense materials like optical fibers but not in air.
Thomson Scattering
Elastic scattering by free electrons, significant in plasma physics but not for FSO systems.
References
[1]http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
[2]https://cefrc.princeton.edu/sites/g/files/toruqf1071/files/Files/2011%20Lecture%20Notes/Alden/Lecture-7-Rayleigh.pdf
https://engineering.purdue.edu/wcchew/ece604f19/Lecture%20Notes/Lect34.pdf
https://www.scratchapixel.com/lessons/procedural-generation-virtual-worlds/simulating-sky/simulating-colors-of-the-sky.html
https://www.soest.hawaii.edu/met/Faculty/businger/courses/notes200/14Optics.pdf