Polarisation By Scattering

What Is Light?

Light is an electromagnetic wave. It is transverse in nature. It is emitted in the vibrating electric field. Thus, the light photons in light waves vibrate in all orientation and are called Unpolarised light beam/waves. Confining their orientation (plane of vibration) to a single plane is called Polarisation. The light thus got is polarised light. This can be achieved by natural phenomena like reflection, refraction, and scattering or artificial methods. In short “Confining the plane of vibration of a light photon to single plane using the principle of scattering of light” is called Polarisation of light by Scattering.

Polarisation By Scattering

Figure: The plane of vibration of light before and after polarisation

How Polarisation By Scattering Works?

When an unpolarised ray of light travels through a medium, it undergoes scattering. The medium is composed of a large number of atoms and ray of light contains number of light photons. When a light ray strikes the atoms in the medium, they transfer their energy to the atoms, and as a result, the electrons in the atoms are set for vibration. These vibrating electrons emit electromagnetic radiation (in visible range i.e., light) in all directions. These radiations strike neighboring atoms and the electrons there also undergo similar mechanism. The radiation emitted by them will have the same frequency as the incident radiation. This mechanism keeps propagating throughout the medium. This absorption and reemission of light radiation cause scattering of light in the medium. The scattered light will have completely polarised light, partially polarised light and completely unpolarized light, depending upon the direction of emitted radiation (explained in the following paragraph). This phenomenon is also observed when sunlight passes through Earth’s atmosphere. The mechanism involved here is well explained below.

Scattering of light

Scattering of light

Figure (1): emitted rays of light after scattering. The emitted rays are completely polarised light, partially polarised light and completely unpolarized light, depending upon the direction of emitted radiation

The light we get from the sun travels through the vacuum before reaching the earth’s atmosphere. Earth’s atmosphere enfolds several layers. Each layer contains air molecules. The light wave radiated from the sun is unpolarised. Once it reaches the Earth’s atmosphere it undergoes scattering. As they are transverse in nature, they vibrate air molecules perpendicular to the direction of their propagation. As a result, electrons in the air molecules starts emitting radiation in the form of light in many directions. Refer to Figure(1) for better visualisation. These light are polarised in the direction perpendicular to the direction of propagation of the original ray.

  • The emitted rays of light which are parallel to the direction/orientation of original rays are completely unpolarised.
  • The emitted rays of light which are perpendicular to the direction of original rays are completely polarised.
  • The rays of light which are scattered in all other directions are partially polarised.

Thus, the sunlight we receive is partially polarised. This is also another reason why the sky is blue! The scattering is well studied under Rayleigh Scattering.

Why Photographers photos have vibrant sky?

The scattered of partially polarised light produce glare in the sky! This challenges the photographer to take the good vibrant photograph of the sky. To overcome this, the photographers use polaroid filter, which on rotation blocks partially polarised light, thereby blocking the glare. This is the photographer’s secret behind vivid blue sky background for the perfect picture.

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Frequently Asked Questions

What are different types of polarisation?

Types of polarisation depends on the orientation of electric field and they are classified as:

  • Linear polarisation where the electric field is limited to the direction of propagation.
  • Circular polarisation where the electric field of light is have two linear components and are perpendicular to each other.
  • Elliptical polarisation where the electric field of light forms an ellipse.

What is polarised light used for?

Following are the uses of polarised light:

  • Polaroid sunglasses use polarised light to reduce the glare.
  • 3D effects can be obtained by using polarised light.

What does light from the Sun is unpolarised mean?

Sun produces light which is unpolarised because the electric field of this light gets distributed in all the directions. When the sunlight passes through the atmosphere, the light gets semi-polarised. For sunlight to be polarised, the angle of incidence and the angle at which polarisation takes place must be equal.

How is linearly polarised light obtained by scattering?

Linearly polarised light can be obtained by scattering. This is possible because when the light is scattered by the air molecules, the incident light and the scattered light are perpendicular to each other, which is visualized as tiny antennae.

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