Rayleigh Scattering - Murthy's Interpretation

Rayleigh Scattering - Murthy's Interpretation

This type of scattering is elastic in nature. This scattering of light by molecules and particulate matter is characterized by their size much smaller than the wavelength of incident light. When light penetrates gaseous, liquid, or solid phases of matter, the result is Rayleigh scattering. Rayleigh scattering intensity has a very strong dependence on the size of the particles. It is proportional to the third power of the diameter of the particle. It is inversely proportional to the fourth power of the wavelength of light. Shorter wavelengths in visible white light (violet and blue) are scattered stronger than the longer wavelengths toward the red end of the visible spectrum. This type of scattering is therefore responsible for the blue color of the sky during the day. Orange colors during sunrise and sunset are also due to the Rayleigh scattering. Rayleigh scattering is the main cause of signal loss in optical fibers. The elastic nature of scattering results in no change from incident light wavelength.

With no change in the wavelength as a result of the elastic nature of Rayleigh scattering, only intensity is higher for larger particles, with 3^rd power proportionality; 3^rd power comes from the cross sectional value of the average incidence of the different rays of light that make up the intensity value. The spheroidal nature of the particles, or approximated to a spheroid, is averaged for all the particles in a unit cross section for the different rays of light, as the intensity of the incident light is scattered more for a dense medium with more particles in unit volume. Only a part of the incident light is scattered, with the remaining part passing through unaffected by the particles.

The Gap in Rayleigh Scattering Research, Understanding and the Equation

It is also important to note and understand that the particles in the medium scattering the incident light are not static, but move around in all three freedoms of directions. The movement of the particles depends on the temperature and pressure, as much as the wind in the case of atmosphere. Atmosphere will also have temperature, pressure and wind gradients in three dimensions. These affect the Rayleigh scattering equation, not explored, discussed or accounted for by scientists doing research in Rayleigh scattering.

Cause for the Elasticity of Rayleigh Scattering

I am explaining here the root cause for elasticity in Rayleigh scattering. Firstly, the particles are much smaller than the wavelength of the incident light in the conditions of Rayleigh scattering. The electron orbits are also much smaller than the wavelengths. The very size and the electric charge fields of the electrons are miniscule compared to the incident light wavelengths. Wavelength is basically and fundamentally the distance travelled by the light ray between successive crests or successive troughs of the electric and magnetic fields of the light. Therefore, there is no field interaction possible between the ray of light and the orbiting electron, meaning no change in wavelength or frequency. No energy quantum transfer from the incident light to the electron means no change in energy of the incident light ray, also meaning no change in frequency or wavelength.

The incident light scatters away in all three dimensions, with its wavelength unaltered. As the particle size is small for the condition of Rayleigh scattering shorter wavelengths scatter more than the longer wavelengths. In Rayleigh scattering in earth's atmosphere blue is scattered much more than the longer wavelengths.

The sunlight arriving directly from the sun therefore appears to be slightly yellow while the light scattered through rest of the sky appears blue. During sunrises and sunsets, the Rayleigh scattering effect is much more noticeable due to the increased density of air and particles near the earth's surface through which sunlight passes.

In contrast, the water droplets which make up clouds are of a comparable size to the wavelengths in visible light, and the scattering is described by Mie's model rather than that of Rayleigh. Here, all wavelengths of visible light are scattered approximately identically and the clouds therefore appear to be white or grey.