Scattering of Light: Factors Affecting and Rayleigh Scattering Theory

Scattering of Light refers to the physical process in which the moving particles are forced to deviate from their straight path due to some localized non-uniformities. Scattering basically refers to the particle-particle collision between atoms, molecules, electrons, protons, and other particles. Scattering happens both for light and sound. This phenomenon is very common in day-to-day life and is seen by everyone. A part of the light is absorbed by the particles of the medium when it passes from one medium to another. The intensity of scattered light depends on the wavelength of the light and the size of the particles.

The most common phenomenon of scattering of light which we see every day are:

1. The red color of the sun during sunrise and sunset.
2. The white color of the clouds.
3. The blue color of the sky.

Scattering is totally different from both reflection and refraction of light. In reflection, the light after striking a polished surface bounces back. But in the scattering of light, the light rays get deviated or scattered in different directions by the medium through which it passes.

Key Terms: Light, Refraction of Light, Atoms, Photons, Scattering of Light, Reflection, Wavelength, Rayleigh Scattering

What is Scattering of Light?

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Whenever a light ray passes from one medium to another, it strikes the particles of that medium.

• Due to this strike, some of the light rays get absorbed by the particles while other gets scattered in all directions.
• The wavelength of the light and the size of the particle which scattered the light ray helps in determining the strength of the scattering.
• Because of the waviness of the line and its interaction with a particle, shorter wavelengths and high frequencies scatter more.
• The more a line waivers, the more likely it is to intersect with a particle.
• Longer wavelengths, on the other hand, have a lower frequency and are straighter, therefore the chances of colliding with a particle are lower.

For example, when light from the sun enters the earth's atmosphere, the rays strike the particles of the air present on the earth. Some of these rays are absorbed by those particles and some are scattered in all directions.

The bending of multicolored light in the afternoon is observed due to the phenomenon of refraction and total internal reflection of light. The wavelength of sunlight forms different colors in various directions. Rayleigh's scattering theory explains the red color of the sun in the morning and the blue color of the sky. The probability of scattering is inversely proportional to the fourth power of the wavelength of the radiation.

If p is the probability of scattering and λ is the wavelength of radiation, then it is given as \(P ⋉ \frac{1}{\lambda^4}\)

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Various forms of light scattering

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The various forms of scattering of light are:

1. Elastic Scattering - When the incident beam of light and the dispersed beam of light have the same energy.
2. Inelastic Scattering - When the incident beam of light and the dispersed beam of light do not have the same energy. Inelastic scattering is further divided into four different types:

Rayleigh Scattering

When radiation of light interacts with molecules and particles in the atmosphere that have a smaller diameter than the wavelength of the incoming radiation, Rayleigh scattering occurs.

• Longer wavelengths scatter more readily than shorter wavelengths.
• Small particles, such as NO₂ and O₂, scatter light with shorter wavelengths (like blue and violet visible light).
• Blue light is more scattered in the atmosphere than longer wavelength red light.
• Incoming sunlight travels a longer distance through the atmosphere at sunrise and sunset.
• The longer journey causes so much scattering of the short (blue) wavelengths that we only see the longer (red and orange) wavelengths of light. 
• The sky would appear black in the absence of particles and scattering.

Mie Scattering

When the wavelength of electromagnetic radiation is similar to the size of air particles, Mie scattering occurs.

• Mie scattering affects photons from the near-ultraviolet to the mid-infrared regions of the spectrum.
• Mie scattering occurs largely in the lower atmosphere, where bigger particles are more prevalent, and it predominates when the sky is overcast.
• The main causes of Mie scatter are pollen, dust, and pollution.
• For Example, the clouds look white due to Mie Scattering.

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Tyndall Effect

The atmosphere of the earth is a diverse combination of microscopic particles. Smoke, tiny water droplets, suspended dust particles, and air molecules are among these particles. The path of a light beam becomes visible when it encounters such small particles. After being diffusely reflected by these particles, the light reaches us.

• The Tyndall effect is caused by the scattering of light by colloidal particles.
• The phenomenon occurs when a fine beam of sunlight enters a room, filled with smoke, through a small aperture.
• As a result, light scattering makes the particles visible.
• When sunlight penetrates through a dense forest canopy, the Tyndall effect is seen.
• Light is scattered by small water droplets in the mist.
• The size of the scattering particles determines the color of the scattered light.
• Light of shorter wavelengths is scattered by very minute particles, while the light of longer wavelengths is scattered by bigger particles.
• The dispersed light may even seem white if the scattering particles are large enough in size.

Raman Effect

The scattering of photons by excited molecules at higher energy levels is referred to as Raman scattering. The photons are inelastically scattered, which implies that the kinetic energy of an incident particle is either lost or gained, with stokes and anti-stokes components.

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Factors on which the Scattering of Light depends

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The factors on which the scattering of light depends are – 

1. The wavelength of the incident light
2. Size of the molecules by which this light is scattered.

 p ∝ 1/(wavelength)⁴

Where p is the probability of scattering of light. So we can say that the light ray with a shorter wavelength has more scattering.

Why the color of the clear sky is blue?

Molecules having a larger size than the wavelength of light experience the light scattering effect differently - the Mie effect. Due to large particles, the light appears white. Therefore the clouds made of water droplets are white.

• The blue color is present majorly in lower wavelengths.
• With the wavelength of light, the scattering efficiency of small molecules present in the atmosphere decreases. 

For red light, the wavelength is more, and the scattering is less in the atmosphere. Therefore, the sun appears red at sunrise and sunset.

Blue Color of the sky 

Things to Remember

• Scattering of Light refers to physical processes in which the moving particles or radiation are forced to deviate from a straight path due to localized non-uniformities.
• Shorter wavelengths and high frequencies of light scatter more.
• The scattering of light is inversely proportional to the fourth power of its wavelength.
• Scattering is responsible for various phenomena such as the blue color of the sky, the reddish appearance of the sun during sunrise, etc.
• On the basis of scattered light, the dispersion can be classified as elastic and inelastic scattering.
• Red light scatters the least while blue light scatters the most.