Polarization of Light

Introduction

Radiation is a process of transmission of energy from one object to another through the medium or space. It can be ionizing or non-ionizing, which depends upon the energy of the radiation. Ionizing radiation has an energy of more than 10eV and can ionize the particles in their path. X - rays, Gamma rays, alpha emissions, and beta emissions come under ionizing radiation. But non-ionizing radiation has not much energy to ionize the particles in their path. Ultraviolet, infrared, visible, and microwave are some examples of non-ionizing radiation.

What is light?

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 polarization?

The wave nature of electromagnetic radiation induces the phenomenon called polarization. Electromagnetic waves contain both electric and magnetic field. which are always perpendicular to each other. As light comes under electromagnetic radiation there are of 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.

Types of polarization

The three types of polarization depending on the motion of longitudinal and transverse waves are elliptical polarization, circular polarization and linear polarization.

Linear polarization

The linearly polarized light waves are waves in which the vibrations of the particle are restricted in one plane which is in the direction of propagation of the wave. Thus there is a restriction in the electric field along one direction. It is also denoted as plane polarized light.

Circular polarization

In circular polarization the angle between the two components of the electric field is 90 degree but the magnitude remains the same. The propagation of waves without changing the magnitude of the electromagnetic field and rotates at a constant rate is circular polarization.

Circular polarisation Dave3457, Circular.Polarization.Circularly.Polarized.Light Without.Components Left.Handed, marked as public domain, more details on Wikimedia Commons

Elliptical polarization

In elliptical polarization the path difference and the magnitude of the linear components of the electric field are different. The special cases of elliptical polarization are linear polarization and circular polarization.

Method of polarization

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 the light changes when a 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 refractionД.Ильин: translation, optimization, Brewsters-angle-ru, CC0 1.0

Polarization by scattering

When light passes through any material the electric and magnetic field 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-emit the light.

Polarization by transmission

In this method materials with special compositions are used as a filter. The light which travels through the polarizer gets polarized to a single direction. One of the components of the electromagnetic wave is cut off. It blocks out one plane out of the two planes. In polaroid material the molecules are positioned in one direction.

Applications

• The polarizers are used in cameras to take a perfect shot.

• Sunglasses come from the fact of polarization and that are 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.

Conclusion

• Light is a form of electromagnetic radiation which is made of photons. They have both electric and magnetic fields. They may be of two types as transverse wave and longitudinal wave.

• In unpolarised light the vibration of particles is in all directions. The restriction given to the vibration of the particles is called polarisation. The particles can vibrate only in one plane.

• The polarization may occur in three ways such as circular, elliptical and plane.

• Polarization of light happens due to four factors such as reflection, refraction, scattering and transmission. It is used in many fields such as spectroscopy, optics, material science etc..

FAQs

1. State Brewster’s law.

This is the angle of the incident light at which the reflected light from the surface is completely polarized is the Brewster’s angle. This is defined by the following formula,

$$\mathrm{\mu=tan \:i}$$

$\mathrm{\mu}$ denotes the refractive index of the material

i denotes the angle of polarization

2. What is Snell's law?

The ratio between the sine of the incident angle and the refracted angle is constant.

$$\mathrm{\frac{sin\: i}{sin\: r}=constant}$$

3. What are the properties of light?

• Light energy is able to pass through the vacuum.

• Light moves at a velocity of about 3 x 108 m/s.

• Light undergoes reflection, refraction, diffraction, scattering, and polarisation.

• The wavelength of light changes when monochromatic light travels from one medium to another. But its frequency does not change.

4. What are the laws of reflection?

The laws of reflections are given below.

• The rays of light that fall on the surface are incident light and the rays that jump off from the surface are called reflected light.

• An non-existing line drawn perpendicular to the reflecting surface at the point of the incident is called normal.

• The incident angle is equal to the reflected angle.

5. What is polarizer and analyser?

The device that is used to polarize an unpolarized is 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.