Scattering of light: Scattering happens when light moves from one medium to another, such as air or a glass of water, a portion of the light is absorbed by the medium’s particles, followed by subsequent radiation in a specific direction. The intensity of scattered light depends on the size of the particles and the wavelength of the light.

For shorter wavelengths, the chance of scattering increases rapidly, and it is inversely proportional to the fourth power of the wavelength of light. In this article, we will learn more about what is meant by scattering of light, the application of light scattering, why the colour of the sky is blue, etc.

What is Meant by Scattering of Light?

It occurs when it travels through a medium containing some obstacles suspended. For example, when sunlight travels through the earth’s atmosphere before reaching the earth’s surface gets scattered by the obstacles like atoms, molecules, dust particles, water droplets, etc., present in the atmosphere.

This is responsible for various natural phenomena like the blue colour of the sky, the reddish colour of the sun during sunrise and sunset, etc. In the scattering of light, the incident light gets absorbed by the molecules, followed by its re-radiation in different directions. Thus, due to scattering, the direction of light changes when it strikes the obstacles present in the medium through which it travels.

Definition of Scattering of Light: It is the phenomenon of bouncing off electromagnetic radiation by the atoms or molecules of the medium through which they are travelling. So, when light travels through a medium, it bounces off in any direction after striking the particles in the medium, as shown in the below diagram:

In the scattering, the light gets absorbed by the molecules of the medium, which later re-radiates the light in different directions. The strength of scattering can be measured by the loss of energy in the light beam as it passes through the medium. The strength of scattering depends on the wavelength of the light besides the size of the particles that cause scattering.

Types of Scattering

The dispersion of light can be classified based on the energy of the scattered light. They are:

1. Elastic Scattering: If the energy of the scattered light is conserved, then such scattering is called elastic scattering. For example, Rayleigh Scattering and Mie Scattering.
2. Inelastic Scattering: If the energy of the scattered light is not conserved, then such scattering is called inelastic scattering. For example, Raman Scattering and Brillouin Scattering.

Rayleigh Scattering

Scattering of Light is basically the change in the direction of light. Lord Rayleigh was the first to study scattering of light from air molecules. He gave the relation,
\(a = \frac{{AV}}{{r{\lambda ^2}}}\)
where
\(A\) is the amplitude of the incident light
\(λ\) is the wavelength of the incident light
\(a\) is the amplitude of the scattered light
\(r\) is the distance from the scattering particle
This relation is applicable when the size of the scatterer is much smaller than the wavelength of light. Now the intensity of the scattered light \((I_s)\) varies directly as the square of the amplitude \((a^2)\) of the scattered light. So the intensity of the scattered light varies inversely as the fourth power of the wavelength of the incident light. Thus, we get the relation,
\({I_s} \propto \frac{1}{{{\lambda ^4}}}\)
Rayleigh further established that the rays do not undergo any change in wavelength on scattering.

Scattering of Light Examples

To define Scattering of Light let’s take examples from our day-to-day life. Some of them are as mentioned below:

Blue Colour of the Sky

The blue colour of the sky can be explained by the Rayleigh scattering of sunlight. The visible light coming from the sun has a wavelength ranging from \(4 \times 10^{-7}\,\rm{m}\) for blue colour to \(8 \times 10^{-7}\,\rm{m}\) for red colour. The light from the sun, while travelling from the earth’s atmosphere, gets scattered by the molecules in the atmosphere.

Here, the size of the molecules that scatter the light from the sun is of the order of \(10^{-10}\,\rm{m}\) which is very less compared to the wavelength of the incident light, so Rayleigh Scattering is valid, and the intensity of the scattered light varies inversely as the fourth power of wavelength of light.

As the wavelength of blue colour is roughly half the wavelength of red colour, therefore, the intensity of scattered blue light is about \(2^4\) times more than that of red light. Due to this, the blue colour dominates, and the sky appears blue.

The White Colour of Clouds

The clouds are located in the lower part of the earth’s atmosphere. The clouds are made up of dust particles, water droplets, ice particles, etc. As the size of these particles is much larger than the wavelength of the incident light, Rayleigh scattering is not valid. Instead, all the wavelengths of the incident sunlight are scattered almost equally.

Therefore, the scattered light coming from the clouds consists of all the wavelengths of visible light. When this light consisting of all the wavelengths of visible light enters the observer’s eye, the observer sees clouds as white.

The Sun Looks Reddish at the Time of Sunrise and Sunset

During sunrise and sunset, the sun is near the horizon. The rays from the sun must travel a larger distance through the dense part of the atmosphere to reach the observer. During this course of the journey of the light rays, most of the blue light gets scattered away. The red colour, which has the highest wavelength among the colours in the visible spectrum, gets scattered the least.

Due to this, the light from the sun during sunrise and sunset entering the observer’s eye consists mainly of light of higher wavelengths like red. Thus, the sun looks reddish at the time sunrise and sunset.

Danger Signals Are Red

Out of the colours of the visible light, the red colour has the highest wavelength. When light passes through the earth’s atmosphere, it gets scattered from the atoms or molecules present in the atmosphere. As the size of these atoms or molecules is much less than the wavelength of the incident light, Rayleigh scattering is valid. As the intensity of the scattered light varies inversely with the fourth wavelength power, the red colour is least scattered.

Due to this, the red colour can be seen from a maximum distance. Red colour can be seen even when there is fog or the atmosphere is unclear. That is why danger signals are red.

Sky Always Appears Dark to the Astronauts in Space

The astronauts in space, when they look up towards the sky, they look at the vacuum in space. There is no atmosphere in space, so there are no particles in space that can scatter light. As no scattering of light takes place, no light enters the astronauts’ eyes in space from the sky, and hence the sky appears dark to them.

Path of Light Becomes Visible in the Canopy of a Dense Forest

When sunlight enters the canopy of a dense forest, tiny water droplets in the mist scatter light. Due to the scattering of light, the particles suspended in the atmosphere shine, and thus, the path of light becomes visible to the observer.

Such a phenomenon is also visible in a smoke-filled room where the smoke particles become visible due to the scattering of light by the particles suspended in the air of the room.

The Wrap-Up

The scattering is the phenomenon of change in the direction of the incident light after striking the obstacles in the medium through which it is travelling. In this article, we have seen why the light of higher wavelengths like red scatters the least, and the light of lower wavelengths like blue scatters the most.

So, the scattering of light is the reason behind many optical phenomena like the blue colour of the sky, the reddish appearance of the sun during sunrise and sunset, etc. In all, we hope this article will have quenched your curiosity to know more about the phenomenon of light.

FAQs related to Scattering of Light

Check frequently asked questions about scattering of light below.

Q.1: What is the scattering of light?
Ans: The scattering of light is the phenomenon of bouncing off of the light in a random direction by the atoms or molecules of the medium through which it is travelling.

Q.2: What are the two types of scattering of light?
Ans: The two types of scattering are elastic scattering and inelastic scattering.

Q.3: What is elastic scattering?
Ans: The scattering of light in which the energy of the scattered light remains conserved is called elastic scattering.

Q.4: What is inelastic scattering?
Ans: The scattering of light in which the energy of the scattered light does not remain conserved is called inelastic scattering.

Q.5: What is Rayleigh scattering?
Ans: Rayleigh scattering is the scattering of light when the size of the scatterer is much less than the wavelength of the incident light, due to which the intensity of the scattered light is inversely proportional to the fourth power of the wavelength of the incident light.

Q.6: Why does the sun appear whitish at noon?
Ans: At noon the sun is nearly overhead, due to which the light from the sun travels a much smaller distance, and the scattering of light takes place much less. Hence, all the colours of the visible light enter the observer’s eye, thus giving a whitish appearance to the sun at noon.