The scattering of light is a very interesting phenomenon as different types of scattering result in very unique observations. The color of the scattered light depends greatly on the size of the particles through which they are scattered. But there are other factors as well that influence the efficiency of scattering.
Rayleigh Scattering vs Mie Scattering
The main difference between Rayleigh scattering and Mie scattering is that in Rayleigh scattering the diameter of the particles (atoms or molecules) is very less than the wavelength of the radiating light source. On the other hand, in Mie scattering, the diameter of the particles is almost the same size as the wavelength of the radiation.
Rayleigh scattering explains a lot of phenomena that we see in our daily life. For example, the blue sky, dark red light during the sunset, and the rainbow are all examples of elastic scattering of light (Rayleigh scattering). In general, we only talk about how light scatters from the gaseous molecules, but Rayleigh scattering can also be explained in some solids and liquids as well.
The Mie theory of scattering is extensively used for solving heat transfer problems in scattering light media. With the help of the concept of Mie scattering, various factors can be calculated such as efficiency factors for scattering, absorption, and extinction.
Comparison Table Between Rayleigh Scattering and Mie Scattering
| Parameters of Comparison | Rayleigh Scattering | Mie Scattering |
| Definition | Rayleigh scattering is defined as the type of scattering in which the diameter of the particles is less than one-tenth of the wavelength. | Mie scattering is defined as the type of scattering in which the diameter of the particle is the same or more than the wavelength of the radiation. |
| Phenomena | Rayleigh scattering can explain phenomena like the blue sky and the red color of the sunset. | Mie scattering can explain phenomena like brownish smog and other behavior of aerosol particles. |
| Particle Size | The particle size in Rayleigh scattering is less than the wavelength. | The particle size in Mie scattering is greater than the wavelength. |
| Type of Particle | The air molecules that are present in the visible part can be explained using Mie scattering. | Scattering of particles of smoke, dust, haze, and cloud droplets in the infrared region can be explained using Rayleigh scattering. |
| Particle Diameter | The diameter of the molecules responsible for Rayleigh scattering is very small (0.001 micrometers). | The diameter of the molecules responsible for Mie scattering is comparatively larger (0.01 micrometers). |
What is Rayleigh Scattering?
Scattering of light is a natural phenomenon that has always existed in nature but it was observed for the first time in the 19th century. A British physicist called Lord Rayleigh noticed the scattering of light for the first time as he was working on the color and polarization of the skylight. He also published two papers on it and thus the phenomenon was named after him. In Rayleigh scattering, two main factors influence the intensity of scattering. They are the wavelength of the radiation and the particle size of the molecules through which they are scattered.
When light particles are propagating, they strike various types of gaseous and liquid molecules that are present in the air. At the time of striking, the electromagnetic field of the incident light transfers the molecular charges which start vibration of the particles. This interaction with the radiation changes the polarization of the incident light to some extent. So, the air molecules absorb some of the energy and then re-emit in various directions. This phenomenon is Rayleigh scattering.
The intensity of Rayleigh scattering is inversely proportional to the fourth power of the wavelength of the incident light. Since blue color has less wavelength than red color, it scatters more and vice versa.
What is Mie Scattering?
Mie scattering gives a generalized solution for a system where a scattering of light takes place by a homogenous spherical medium. And this medium should have a refractive index different from that of the medium through which the light is traversing. Unlike Rayleigh scattering, Mie scattering is not a physically independent phenomenon rather it is a solution to Maxwell’s equations for situations where the phase of the incident angle can change within the dimension of the scattering particles. Mie scattering is more commonly known as Mie solution and it is named after Gustav Mie, a German physicist.
Mie scattering is also known as aerosol particle scattering takes place in the atmosphere below 1,500 feet. In Mie scattering, the diameter of the spherical particles through which the light is scattered is approximately equal to the wavelength. Several studies have been conducted related to Mie scattering to find more effective approaches for the Mie solution. The mathematics behind Mie scattering is quite complex to fathom for everyone.
Some of the readily available types of particles in our environment that cause Mie scattering are smoke, dust, water vapor, aerosol, pollen, etc. During cloud formation, several aerosol particles diffuse with each other, and due to this, the color of the cloud appears to be white.
Main Differences Between Rayleigh Scattering and Mie Scattering
- In Rayleigh scattering, the scattering efficiency varies inversely with the fourth power of the wavelength whereas, in Mie scattering, the efficiency is not strongly dependent on wavelength.
- Rayleigh scattering is used for laser imaging in various types of gases whereas Mie scattering is suited for measuring flow velocities using Particle Image Velocimetry (PIV).
- Rayleigh scattering is a weaker scattering-type as compared to Mie scattering.
- The range of the particle size diameter in Rayleigh scattering is much smaller than that of the Mie scattering.
- Witnessing various colors (blue sky, red sunset, rainbow, etc.) is a result of Rayleigh scattering whereas the white appearance of cloud is due to Mie scattering.
Conclusion
When we observe natural colorful phenomena around us every day, most of us barely think about it. But the physics and mathematics behind explaining these events are vast. Not a single person came up with theories and solutions in s single day. Every advancement is made depending on the work or failure of other scientists. When Rayleigh scattering was observed, it was also based on the earlier studies made in the Tyndall effect. So, the theories were similar and one established hypothesis raised an idea for another. Rayleigh scattering is much simpler to understand than Mie scattering as the latter one is much more complicated mathematically.
References
- https://www.osapublishing.org/abstract.cfm?uri=ao-20-4-533
- https://www.osapublishing.org/abstract.cfm?uri=AO-43-9-1951