Scattering of Light

The scattering of light is an important part of our daily life, although we didn’t realize its importance. Scattering of light is different from reflection, as in reflection the radiation is deflected in one direction while in scattering every object or particle can scatter light and illuminates them in all directions. When a parallel beam of light passes through any particle present in air or gas; the particles present in the air scatter the light beam in all directions besides its incident direction. This phenomenon of, light striking the particles present in the air and after absorbing some light it radiates it in different directions except its incident direction, is called as “Scattering of light”. The loss of energy in a light beam after scattering can be calculated by the strength of scattering, its value depends on the wavelength of the light and the size of the particle which causes scattering. 

This process of scattering of light can also be shown with the help of an example of light rays falling from the sun, as when the rays of the sun enter the earth’s atmosphere there are various small particles present in the air. These particles collide with the sun rays falling and while absorbing some light making them scatter/ deflect in different directions besides its incident direction. Basically, we can say that a ray of light is deflected from its straight path due to some irregularities present in the medium, particles, or due to the interference between the two media.

When the light passes from one medium, say water, to another medium, say air, some of the light is absorbed by the first medium before it scatters into the second medium. The amount of light absorbed in the first medium and the amount of light scattered varies based on the wavelength and intensity of the light. When the wavelength of the light ray is less, it has more waves and the chance of light particles colliding with the other particles in the medium are more. So the scattering of light is generally more. On the other hand, when the wavelength of the light ray is more, the frequency of the wave is less and there are fewer chances of light particles getting scattered.

There are several ways how the scattering of light takes place, but two main examples are:-

• Random reflection from a rough surface

• Reflection (refraction or diffraction) through impurities present in the volume.

Random Reflection from a Rough Surface

All surfaces are rough; the roughness of the surface is related to the wavelength of the light ray. As the surface has more roughness absorbs more energy from the light ray and will scatter it in different directions depending on the wavelength of the light ray. The rough surface of a car or pieces of jewelry can be taken as a good example of scattering through a rough surface.

Reflection Through Impurities present in Volume

This type of scattering through impurities present in the volume helps in the medical types of equipment. Thompson mechanism, which is also known as elastic scattering, is a good example of this type of scattering which is used in medical X-ray photographs. This type of scattering occurs where light is scattered by charged particles by leveling the energy of the incident radiation and the rest energy of the charged particle present. The passing of light rays through any liquid in which light beam scatters only with the tiny fat droplets is a good example of this type of scattering.

Single and Multiple Scattering

When the scattering of light takes place with only a single localized scattering center then the phenomenon is known as single scattering, it is generally treated as a random phenomenon as it has only a single scattering center that can do a single scattering event at a particular time. While that scattering takes place with many localized scattering centers, a large number of localized centers involves a large number of combined results which give more number of scattered light to the observer. The single type of scattering is not always random, as they can be intentional sometimes in the case of a laser beam, which can be well controlled for scattering to a single point for an instance, along with the radar scattering where the targets tend to be microscopic in size.

Types of Scattering of Light

1. Rayleigh or selective scattering

2. Mie scattering

3. Electromagnetic scattering

Rayleigh or Selective Scattering

Rayleigh scattering of light is a type of elastic scattering as the particle from which the scattering has to be done depends on the wavelength of light. Depending on the wavelength, certain particles are more effective than others which scatter light having more wavelength, as the particles like molecules of oxygen, nitrogen having small size scatter light with a shorter wavelength (blue or violet) in different directions. The blue sky on a clear sunny day is also the result of Rayleigh scattering by the air molecules. 

The blue light having a shorter wavelength appears to scatter from the upper atmosphere about 10 times which is much larger than the red light having a larger wavelength. Hence, the blue light having a shorter wavelength collides with the air molecules and is scattered to the eyes seeing the sky, making it blue. On the other hand, the red light with a higher wavelength goes largely unscattered to the sky. Scattering of optical signals through optical fibers is also included in this type of scattering.

Mie scattering

Mie scattering is also a type of elastic scattering mechanism, in which the size of the molecules is greater than the wavelength of the light which results in the non-uniform scattering of light. This type of scattering is not much dependent on wavelength as the size of the molecule which scatters light is more important. Due to this process, the clouds having water droplets look white. 

The scattering efficiency of the small molecules becomes less in the atmosphere with the wavelength of the white light. This shows that the light ray which enters the clouds get scattered by water droplets for all wavelength and no light of visible wavelength is left in the cloud making the cloud look white. When the clouds become full of water droplets no light is taken in the cloud for scattering making it looks darker. The light rays falling on the earth’s surface results in different types of scattering, making the color of the sky blue and making the color of the clouds white. The white color of fog and clouds both are the results of “Mie scattering”.

Electromagnetic Scattering

It is the most common form of scattering as it includes electromagnetic waves. It generally includes two types of scattering which are elastic light and inelastic light scattering. Elastic light scattering includes Rayleigh scattering or Mie scattering while inelastic scattering includes Raman scattering, inelastic x-ray scattering, Compton scattering, and Brillouin scattering.

The intensity of the light scattered from the molecules generally depends on two factors which are the wavelength of the light to be scattered and the size of the molecules due to which the light falling on the earth’s surface is scattered. The light ray falling on the earth’s surface having a shorter wavelength and high frequency tend to scatter more because of the intersection with the particles and the waviness of the line. While the light rays falling on the earth’s surface having a low frequency and longer wavelength tend to scatter less because they move in a straighter path which makes the possibility of colliding with the particles less.

The probability of scattering and λ the wavelength of the light can give the relation as follows:

\[P\alpha \frac{1}{\lambda ^{4}}\]

Where P is the probability of scattering

And, λ is the wavelength of the light.

The above relation between the probability of scattering and the wavelength of the light shows that the probability of scattering is inversely proportional to the fourth power of the wavelength and the probability of scattering will be higher for the shorter wavelength of the light.