Scattering of Light is a fascinating phenomenon that occurs when light travel through different mediums and is scattered by the particles of the medium or other irregularities. Light is nothing but energy which can be either considered waves or particles (dual nature of light) and travels in a straight line and as light reaches our eyes then we can see everything around us. The scattering of light has significant implications in daily life, from the colour of the sky to the colour of clouds are all examples of scattering of light. As there is various terminology involved in the explanation of the scattering of light such as refraction. Thus, let’ understanding refraction in brief before the detailed explanation of scattering and its types.

Refraction of Light Definition

When light or any other wave passes from one medium to another medium than the deflection of the path of light; due to the difference in the refractive index of the mediums, is called refraction of light.

One of the most well-known occurrences of refraction is light, although other waves, such as sound and water waves, can also refract due to the same reason.

Scattering of Light Definition

When light passes through one medium, some portion of the light is absorbed by the medium’s particles when it goes to another medium, such as air or a glass of water then some intensity of light is radiated in the direction of the coming light but some part of it defected to different direction based on the wavelength of the constituent light. This phenomenon is known as a scattering of light. 

In the afternoon, you may observe the bending of multicoloured light due to refraction and total internal reflection. Sunlight contains different colours of light, each with a different wavelength that can be separated as it passes through the atmosphere. Rayleigh’s scattering theory can help explain why the sky appears blue and why the sun appears red during sunrise or sunset. The theory states that shorter wavelengths of light, such as blue light, are scattered more easily by atmospheric particles, while longer wavelengths, like red light, pass through with less scattering.

Factors Affecting Scattering of Light

Shorter wavelengths and higher frequencies scatter more due to the waviness of the line and its interaction with a particle. A line is more likely to collide with a particle if it is wavy. Longer wavelengths, on the other hand, have a lower frequency and are straight, which means they have a smaller likelihood of colliding with a particle. Therefore, the scattering of light depends upon the size of the particle and the wavelength of the light. 

Size of the particles 

The colour or wavelengths of the particle scattered depends upon the size of the particles such as 

• Tiny particles scatter light of a shorter wavelength.
• Large particles scatter light of a longer wavelength.

Wavelength of the Ray 

Scattering is inversely proportional to the wavelength.

Scattering ∝ 1/λ

where λ denotes the wavelength of the ray. 

As there is inverse proportionality of the wavelength and scattering this means that the light with a higher wavelength scatters more then than light with fewer wavelengths.

Different forms of Scattering of light

Light dispersion takes place in many forms that are discussed below:

• Elastic Scattering
• Inelastic Scattering 

Let’s understand these topics in detail.

Elastic Scattering

When the energy of the incident and scattered beams of light is the same, then the scattering is called elastic scattering.

Inelastic Scattering 

When the energy of the incident beam of light and the dispersed beam of light differs. Inelastic scattering is further classified into four types:

• Rayleigh Scattering
• Mie Scattering
• Tyndall Effect
• Raman Effect

Rayleigh Scattering

When radiation (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 when compared to shorter wavelengths. Small particles, such as NO₂ and O₂, scatter light with shorter wavelengths (like blue and violet visible light). Red light, which has a longer wavelength, scatters more in the atmosphere than blue light. Incoming sunlight travels a larger distance through the atmosphere at sunrise and dusk. Due to the longer route dispersing the short (blue) wavelengths, we only see the longer (red and orange) wavelengths of light.

Mie Scattering

When the wavelength of electromagnetic radiation is similar to the size of air particles, Mie scattering occurs. Mie scattering affects photons in the near-ultraviolet to mid-infrared regions of the spectrum. Mie scattering occurs largely in the lower atmosphere when the sky is overcast, where bigger particles are more frequent. Mie scattering is mostly caused by pollen, dust, and pollution. For example, Mie Scattering makes the clouds appear white.

Tyndall Effect

A variety of tiny particles make up the Earth’s atmosphere. Smoke, small water droplets, suspended dust particles, and air molecules are examples of these particles. The path of a light beam becomes visible when it collides with such little particles. After being diffusely reflected by these particles, the light reaches us. The Tyndall effect is caused by colloidal particles dispersing light. The phenomenon occurs when a fine beam of sunlight enters a smoke-filled room through a small hole. The particles become visible as a result of light scattering. When sunlight penetrates through a dense forest canopy, the Tyndall effect is noticeable. Light is scattered by little water droplets in the mist. The size of the scattering particles determines the hue of the dispersed light. Very small particles scatter shorter wavelength light, while larger particles scatter longer wavelength light. The dispersed light may appear white if the scattering particles are large enough.

Raman Effect

Raman scattering is the scattering of photons at higher energy levels by stimulating molecules. The incident particle’s kinetic energy is either lost or acquired, with Stokes and anti-Stokes components, because the photons are inelastically scattered.

Applications of Scattering of Light

Why Is the Colour of the Clear Sky Blue?

Blue colour has a shorter wavelength compared to red colour. Since we know that, scattering ∝ 1/λ. Hence, the blue colour gets scattered most by tiny minute particles in the atmosphere during the daytime. The atmosphere has the presence of various gases such as Nitrogen (N₂) and Oxygen (O₂). These gas molecules are very small in size and form a colloidal (Gas-in-Gas solution). Small-sized particles scatter rays of shorter wavelengths and blue colour being of shorter wavelength gets scattered more strongly and gives a blue appearance to the sky. The space appears dark to astronauts, as there is no atmosphere. Without the atmosphere tiny particles aren’t there to scatter light, hence giving a dark appearance.

The image added below shows the blue colour of the sky.

Red Color of Danger Signals 

The wavelength of red colour is longer when compared to other colours of the spectrum (seven colours are formed due to refraction through a prism). As rays of longer wavelength are least scattered by dust and smoke particles, red colour reaches far away distances and would help danger signals to reach faster and to more distant places. All other colours scatter away during the night, and the red colour reaches our eyes.

Red appearance of Sun during Sunrise and Sunset

During sunrise and sunset, the rays have to travel a longer distance through the layers of the atmosphere because they are very close to the horizon. Therefore, all other colours except the red colour scatter away, and the red colour remain. Most of the red light, which is the least scattered by the particles, enters our eyes. Hence, the sun and the sky appear red. At noon, the sun appears white as less of the blue light gets scattered.

White Colour of Clouds

The clouds, positioned close to the Earth’s surface, are composed of dust particles and water molecules larger than the visible light wavelength. These particles uniformly scatter all colors of the incoming sunlight, resulting in the clouds appearing white when the scattered light reaches our eyes.

Tyndall Effect

The Tyndall effect, also known as Tyndall scattering, refers to the dispersion of light by particles like dust, smoke, and water droplets that are suspended in the air in a colloidal state. This phenomenon was first described by John Tyndall in 1859.

Sample Questions on Scattering of Light

Question 1: What colour does the clear sky appear to be during the day? Give an explanation.

Solution:

Blue is the colour of the sky throughout the day. This is because of the size of air molecules and other fine particles in the atmosphere is smaller than the wavelength of visible light. Due to this, these particles scatter light rays of shorter wavelengths at the blue end more efficiently than light rays of longer wavelengths at the red end. That is why the scattered blue light gives us the impression of a blue sky when it enters our eyes.

Question 2: What is meant by the scattering of light? 

Solution:

Light scattering is the spreading of light in different random directions. Light scatters when it encounters various types of suspended particles along its path .The colour of scattered light is determined by the size of scattering particles in the environment.

• The larger dust and water droplets in the atmosphere scatter light with longer wavelengths, giving the dispersed light a white appearance.
• The very small particles in the environment, such as air molecules, scatter the blue light contained in the white sunlight.

Question 3: Why does the sun appear reddish early in the morning?

Solution:

At daybreak, the sun rises near the earth’s horizon (early in the morning). Light from the sun near the horizon must travel through vast layers of air and a great distance via the earth’s atmosphere before reaching our sight. The particles in the atmosphere scatter most of the blue light rays with shorter wavelengths near the horizon. As a result, we are exposed to red light with longer wavelengths. As a result, the sun takes on a crimson colour.

Question 4: Why red color is used to make a danger signal or sign?

Solution:

When red collides with small fog and smoke particles, it scatters the most since it has the longest wavelength (visible spectrum). As a result, we can see the red colour clearly even from a great distance

Question 5: On a foggy day, why does the driver use orange lights instead of white lights?

Solution:

When a driver uses white light while driving in fog, the tiny droplets of water scatter a lot of blue light. This diffused blue light reduces visibility and makes driving a challenge. Orange light does not scatter due to its larger wavelength, allowing the motorist to see clearly.

Scattering of Light – FAQs

What is Scattering of Light?

When light travels from one medium to another medium some of it is absorbed by the medium and then some part of the absorbed energy is deflected by particles in different directions due to he different wavelengths of the constituent of the light. This process of deflection of light is called scattering of the light.

What Causes the Scattering of Light?

The scattering of light is caused by the interaction of the particles and irregularities of the medium from which light passes through and the size of the particles defines the extent of scattering in the medium.

What are the Different Types of Scattering of Light?

There are various type of the different scattering of light happens, some of which are as follows:

• Elastic Scattering
• Inelastic Scattering 
  • Rayleigh Scattering
  • Mie Scattering
  • Tyndall Effect
  • Raman Effect

How does Scattering Affect the Colour of the Sky?

The blue color of the sky is due to the atmosphere present on the earth as particles of the atmosphere absorb the longer wavelengths such as red and scatter the longer wavelengths such as blue. Thus, blue color enters the eye of the humans causing the color of the sky to be blue.

How does Scattering Affect Visibility?

As some intensity of the light is scattered in the various direction of the atmosphere and the light entering our eyes have less intensity thus, affects the visibility of far away things.