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Polarisation by Scattering
Introduction
Light is electromagnetic radiation and is the only part of the electromagnetic spectrum that is visible to the human eye. Photon is the basic unit of light. All electromagnetic energy is made up of photons. They are always neutral. That is, they have no electric charge. They do not decay on their own. Within the electromagnetic spectrum, the wavelength range of the light that is visible to the human eye is 400-700nm.
What is scattering?
When light travels from one medium to another such as air or water, The particles in the medium absorb the light that passes through it and gets radiated in different directions. This is known as scattering.
For example, when the sunlight passes through the atmosphere and reaches the earth, the particles in the atmosphere absorb the light and scatter them in different directions.
This is responsible for the blue colour of the sky.
When the light passes through the medium the particles bounce off the light. This is known as scattering.
The size of the particles and the wavelength of the light are responsible for the intensity of scattering.
What is polarization?
The wave nature of electromagnetic radiation induces the phenomenon called polarization. Electromagnetic waves contain both electric and magnetic fields. which are always perpendicular to each other. As light comes under electromagnetic radiation there are two types such as transverse wave and longitudinal wave. Transverse waves are waves in which the vibration of particles is in a perpendicular direction to the direction of motion of the wave. Longitudinal waves are waves in which the vibration of particles is in a parallel direction to the direction of motion of the wave. The property of polarization is applied to transverse waves. In unpolarized light, that is the light from the sun or light from a bulb the vibrations of particles are possible in all possible directions. Polarization made the particle vibrate in a single plane. The methods that are used to polarize the light waves are given below.
Polarization by reflection
A non-metallic surface reflects a polarized light if the unpolarized light falls on it at a particular angle. The magnitude of polarization is regulated by the incident angle and the surface of the non-metal. Most of the light that is reflected from the surface is polarized and they are parallel to the propagation light.
Polarization by refraction
The direction and speed of light change when light travels from one medium to another. Most of the light that is refracted from the surface is unpolarized and has only one or two polarized light. They are partially polarized and parallel to the propagation of light.
Polarisation by reflection and refraction Д.Ильин: translation, optimization, Brewsters-angle-ru, CC0 1.0
Polarization by transmission
In this method materials with special compositions are used as a filter. The light which travels through the polarizer gets polarized in a single direction. One of the components of the electromagnetic wave is cut off. It blocks one plane out of the two planes. In polaroid material, the molecules are positioned in one direction.
Working of polarisation by scattering
When light passes through any material the electric and magnetic fields on the light are radiated and scattered off from the surface. The absorption and re-emission of the light made the light into partially polarized light. Every molecule or atom in the material absorbs the light and re-emits the light. When an unpolarised light passes through the medium it gets absorbed and re-radiated in different directions being partially polarised or completely polarised.
Applications
The polarizers are used in cameras to take a perfect shot.
Sunglasses come from the fact of polarization and which is used to reduce glare.
The formation and screening of three-dimensional movies are done by the concept of polarization.
It is used in the study of earthquakes, seismology and infrared spectroscopy.
In plastic industries, the stress analysis is done by polarization.
It helps to differentiate the transverse waves and longitudinal waves.
Testing of chirality of the organic compound is done by polarization.
The various colours of the sky happen due to the scattering of light.
Conclusion
Light is electromagnetic radiation which contains both electric and magnetic fields.
In unpolarised light, the vibration of particles is in all directions. The restriction given to the vibration of the particles is called polarization. The particles can vibrate only in one plane.
Polarisation of light happens in three ways and they were discussed in detail above.
The light that passes through the medium gets absorbed by the scattered particle that is present in the medium.
When unpolarised light passes through the medium and is scattered off in different directions it is being partially or completely polarised.
They are responsible for the colour of the sky, sun during sunset and sunrise and rainbow.
Their applications were also given above.
FAQs
1. What are polarizers and analysers?
The device that is used to polarize an unpolarized is a polarizer. The rotation of the polarizer is possible in 360 degrees. The device that is used to analyse whether the light is plane polarized or not is an analyzer. The analyser can be moved only in and out of the light’s path.
2. Why does the sky look blue but the clouds are white?
Light that passes through the earth’s atmosphere has a wavelength in the range of 400 nm to 800 nm.
The size of particles that are present in the earth’s atmosphere is around 10-10 m, which is smaller than the wavelength of the light.
So as per Rayleigh scattering, the intensity of the scattered light is inversely proportional to the wavelength of the incident light.
As the wavelength of the blue is half of the wavelength of red the intensity of scattered blue light is higher than red. So. The sky looks blue.
But the particles in clouds are larger than the wavelength of the incident light as clouds are in the lower atmosphere; the scattered light contains all wavelengths equally.
So they appear white.
3. Astronauts see only dark clouds in space. Why?
In space, there is no atmosphere which means that there are no intermediate particles in space for the scattering of light. As there is no possibility of scattering of light from the sky it looks dark for the astronauts.
4. Why are the sound waves not polarized?
Generally polarisation happens only in the transverse waves only. As the sound waves are longitudinal waves they cannot be polarized.
5. What happens if there is no scattering?
If there is no scattering then,
The formation of rainbows is not possible in the sky.
The sky always looks black.
The sun always looks white even during the sunset and sunrise.
