What is Rayleigh Scattering : Scattering of Light & It Losses

Lord Rayleigh (12th November 1842) was discovered rayleigh scattering. We know the phenomenon of light that is reflection and refraction. The particles in the atmosphere called scatter because when the light is entered into the atmosphere then these particles will be scattered into lights. This phenomenon of refraction can be called as a scattering of light. There are two types of scatterings like elastic and non-elastic. The rayleigh, Mie, and non-selective scatterings are elastic scatterings and the Brillou, Raman, In-elastic X-ray, Compton is the in-elastic scatterings. In this article, one type of elastic scattering namely Rayleigh is discussed briefly.

What is Rayleigh Scattering?

Definition: The rayleigh is the scattering of the molecules by the gas in the atmosphere of the earth. The scattering strength depends on the light wavelength and also on the particle size. Due to the compositional variations, the rayleigh or linear scattering is caused.

Scattering of Light

We have crossed some wonderful phenomena in our daily life, such as the blue color of the sky, the color of the water in the deep sea, the reddening of the sun at sunrise and the sunset, etc. When a beam of light falls on an atom it causes the electron in the atom to vibrate. The vibrating electrons, in turn, it re-emits the light in all direction and this process is called scattering.

The earth atmosphere contains air molecules and other tiny particles when light from the sun passes through the atmosphere, it gets scattered by a large number of particles in the atmosphere. According to the Rayleigh Scattering Law (RSL), the intensity of scattering light varies inversely as the fourth part of the wavelength of height (1/h⁴). Compared to the longer wavelengths the shorter wavelengths are scattered more. The linear scattering diagram is shown in the below figure.

According to the RSL, the blue color light is scattered more than the red light because, for this reason, the sky appears in blue. At sunrise and sun-set the rays from the sun travel a large part of the atmosphere. Therefore, most of the blue light is scattered away and only the red light reaches the observer. Hence the sun appears red at sunlight and sunset.

In the case of light scattering, nearly all the scattering light is observed at the same frequency as the incident radiation. This phenomenon is called elastic or rayleigh or linear scattering, however, the great Indian physician Dr.C.V.Raman observed that the scattering of light has discrete frequencies above and below the incident frequency in 1928. The applications of rayleigh or linear type are lidar (light detection and ranging), weather radar, etc.

Rayleigh Scattering Losses

The scattering losses exist in optical fibers because of microscopic variation in the material density and the composition. As glass is composed of randomly connected networks at molecular and several oxides like silicon oxide, GeO₂ etc. These are the major use of composition structure fluctuation, these two effects result in variation in refractive and rayleigh type of scattering of light.

The scattering lights due to small localized changes in the refractive index of the core and cladding material. These are the two causes during the manufacturing of fibers. The first is due to the slight fluctuation in the mixing of ingredients and the other cause is a slight change in density as the solidifies. The below figure shows graphically the relationship between wavelength and scattering loss of rayleigh.

When a light ray strikes such zones, it gets scattered in all direction, the scattering loss for single component glass is given by

Α_scat = 8π³ / 3λ⁴ (n² – 1) ² K_B T_f B_T

Where n = Refractive index

K_B = Boltzman’s constant

B_T = Isothermal compressibility

T_f = Frictive temperature
Based on the dimensionless size parameter, the scattering of light is divided into three domains and it is defined as

A = πDp/ λ

Where  Dp= Circumference of a particle

λ = Incident wavelength radiation

The Rayleigh is proportional to the and the P(r), A(r), and r. The mathematical expression is given by

α= α_R+ α_IM  +α_OH+α_IR+α_UV+α_IM

Where α_R=RSL

α_IM =Imperfection loss

α_OH = Absorption loss

α_IR = Infrared absorption loss

α_UV = Ultraviolet absorption loss

α_IM =Other impurities absorption loss

An α_IR (infrared absorption loss) is mathematically expressed as

α_IR =C exp(-D/λ)

Where ‘C’ is the coefficient and D is dependent on materials

The loss is proportional to the λ⁴ and to the P(r), A(r), and r. The mathematical expression is given by

α_R = 1/λ⁴ ∫₀^+∞  A(r) P(r)rdr / ∫₀^+∞  P(r)rdr

Where A(r) =  Linear scattering coefficient

P(r) = Light intensity propagation

‘r’ = Radial distance

This is the theory of linear scattering loss.

Difference Between Rayleigh and Mie scattering

The difference between these two is discussed below.

Rayleigh Scattering in Optical Fiber

The optical fiber is thin, flexible, and transparent of optically pure silica glass and plastic. The optical fibers are faster, impervious to electromagnetic interference, cannot catch on fire and the signal loss is less. When a beam of light that carries signals travels from the fiber optics, then the strength of the light becomes lower, this loss of light power is generally called attenuation. An attenuation has to be a top priority for many engineers to consider selecting and handling fiber optics.

All most all objects scatter lights, that means the reflected light that illuminates them in all direction. The rayleigh or linear scattering is caused by the interference with particles smaller than the wavelength of the light. The light travels through the fiber interacts with the particles and then scattered in all directions, it causes energy losses and attenuation during the data transmission. This is the theory of Rayleigh or linear scattering in optical fibers.

FAQs

1). What causes Rayleigh or linear scattering?

The causes of rayleigh or linear scattering are, it results from inhomogeneities in the cladding and core. The density &compositional variations and fluctuation in refractive index are the problems that occur because of the inhomogeneities.

2). Who discovered Rayleigh scattering?

John William Strut was discovered.

3). What is the difference between Rayleigh and  Mie scattering?

In the Rayleigh or linear scattering, the size of the scattering particles is smaller than the radiation wavelength and in the Mie-scattering the size of the scattering particles and the wavelength of radiation is the same.

4). What are the three types of scattering?

The three types of scattering are the rayleigh, non-selective scattering, and the Mie scattering.

5). What is the Rayleigh ratio?

The rayleigh ratio is one of the parameters which is used for the light scattering measurements.

In this article, an overview of Rayleigh Scattering or linear scattering, scattering of light, scattering losses, and the difference between Rayleigh and Mie scattering are discussed. Here is a question for what are the causes of Mie scattering?