Optics- Polarisation Of Light - Linearly Polarised Light Or Plane Polarised Light

Optics: Polarisation of Light - Linearly Polarised Light or Plane Polarised Light

Light is an electromagnetic wave.
It consists of electric and magnetic fields oscillating perpendicular to each other and to the direction of propagation.
When light waves vibrate in only one plane, it is called linearly polarised light.
The plane in which the electric field oscillates determines the direction of polarisation.
Polarisation can be achieved using various methods.

Polarisation by Reflection

When light is incident on a non-metallic, transparent surface at a specific angle called Brewster’s angle, it gets polarised.
The reflected light becomes partially or fully polarised, depending on the angle of incidence.
The plane of polarisation is parallel to the reflecting surface.
This principle is used in polarising sunglasses.

Polarisation by Scattering

When sunlight passes through the atmosphere, it undergoes scattering by particles and molecules present in the air.
This scattering tends to polarise the light.
The scattered light is partially polarised, with its electric field mainly vibrating in a specific direction.
This is why the sky appears blue, as the blue light is scattered more than other colors.

Polarisation by Use of Polaroid

Polaroid is a material that can transmit only light waves vibrating in a particular direction.
It is made up of long-chain molecules aligned parallel to each other.
These aligned molecules absorb light waves vibrating in their direction and allow only the perpendicular vibrations to pass through.
Polaroid filters are commonly used in cameras and sunglasses.

Polarisation by Double Refraction

Certain crystals exhibit a property called double refraction or birefringence.
When unpolarised light passes through such crystals, it gets split into two polarised rays traveling with different velocities.
The two rays have different refractive indices and vibrate in different planes.
This phenomenon is utilized in optical devices like polarisers and analyzers.

Malus’ Law of Polarisation

Malus’ law relates the intensity of polarised light transmitted through a polariser to the angle between the polariser and the plane of polarisation.
It states that the intensity of polarised light transmitted by a polariser is proportional to the square of the cosine of the angle between the polariser and the plane of polarisation.
The mathematical representation of Malus’ law is: $ I = I_0 \cos^2\theta $ , where $ I $ is the intensity of transmitted light, $ I_0 $ is the initial intensity, and $ \theta $ is the angle between the polariser and plane of polarisation.

Polarisation by Optical Activity

Certain substances possess the property of rotating the plane of polarisation of light passing through them.
This property is known as optical activity.
Optically active substances are classified as either dextrorotatory or levorotatory, depending on the direction of rotation.
This property is used to determine the concentration of enantiomers in chemistry and in the pharmaceutical industry.

Applications of Polarisation

Polarisation of light has several practical applications:
- Polarising sunglasses reduce glare and improve visibility.
- LCD screens in televisions, computer monitors, and smartphones use polarisers to control light transmission.
- 3D movie glasses use polarisation to separate the left and right eye images.
- Optical filters and polarisers are used in photography to control light intensity and reduce reflections.

Summary

Light waves can be polarised by reflection, scattering, use of polaroid, double refraction, and optical activity.
Linearly polarised light consists of electric and magnetic fields oscillating in a single plane.
Malus’ law describes the transmission of polarised light through a polariser.
Polarisation has various applications in eyewear, displays, photography, and optical instruments.
Understanding polarisation is essential in the study of optics and the principles of light propagation.

Polarisation by Transmission through Crystals

Certain crystalline materials, like calcite, can polarise light by transmitting it through their crystal structure.
These materials have different refractive indices for light vibrating in different planes.
As the light passes through the crystal, its components vibrating in different planes are separated, resulting in polarisation.
This phenomenon is crucial in understanding the working of polarisers and analysers.

Polarisation by Dichroism

Dichroic materials have different absorption coefficients for light polarised in different directions.
When unpolarised light passes through a dichroic material, it gets partially polarised based on the absorption characteristics of the material.
Examples of dichroic materials include certain types of glass and some plastics.

Circular Polarisation

Circularly polarised light consists of electric and magnetic fields that rotate as the wave propagates.
It can be thought of as the combination of two perpendicular linearly polarised waves with equal amplitudes and a phase difference of 90 degrees.
Circular polarisation can be achieved by passing linearly polarised light through a quarter-wave plate or by using certain crystals and special filters.

Huygens’ Principle

Huygens’ principle states that every point on a wavefront can be considered as a source of secondary wavelets.
The secondary wavelets form spherical wavefronts that propagate in the forward direction.
This principle helps explain various optical phenomena, including diffraction and interference.

Brewster’s Law

Brewster’s law relates the angle of incidence and the angle of polarisation for light reflecting off a non-metallic surface.
According to this law, the tangent of the angle of polarisation is equal to the refractive index of the second medium.
Mathematically, it can be represented as $ \tan \theta_p = \frac{\mu_2}{\mu_1} $ , where $ \theta_p $ is the angle of polarisation, $ \mu_1 $ is the refractive index of the incident medium, and $ \mu_2 $ is the refractive index of the second medium.

Polarisation and Transverse Wave Nature

Polarisation is possible only for transverse waves.
Transverse waves are waves in which the displacement of the medium is perpendicular to the direction of wave propagation.
Sound waves, which are longitudinal waves, cannot be polarised.
This distinction is important in determining the possibility of polarisation in different types of waves.

Analyser

An analyser is an optical device that can determine the state of polarisation of light.
It transmits a specific plane of polarised light and blocks all other planes of polarised light.
It is often used in conjunction with a polariser to observe, measure, or analyze polarised light.

Polarisation and 3D Glasses

3D glasses use polarisation to create the illusion of depth in movies and other media.
The glasses have different polarisers for each eye, which separate the left-eye and right-eye images.
By presenting slightly different images to each eye, the brain perceives the depth and three-dimensional effect.

Polarisation and Liquid Crystals

Liquid crystals are materials that exhibit properties of both liquids and crystals and have unique optical properties.
They can be electrically controlled to change their light transmission properties.
Liquid crystal displays (LCDs) utilize the polarising and light-controlling abilities of liquid crystals for electronic visual displays.

Applications of Polarisation in Communication

Polarisation is used in certain radio and wireless communication systems.
By transmitting and receiving signals using different polarisations, interference can be reduced and multiple channels can be utilized.
This technique is known as polarisation diversity and is commonly used in satellite communication and cellular networks.

Uses of Polarisation in Photography and Astronomy

Polarising filters are used in photography to reduce reflections and enhance color saturation.
They can be rotated to adjust the amount of polarisation and eliminate unwanted reflections.
In astronomy, polarisation is used to study the magnetic field of celestial objects and to analyze light from distant galaxies.

Optically Active Substances

Optically active substances rotate the plane of polarisation of light passing through them.
Chiral molecules, which lack internal symmetry, exhibit optical activity.
Examples include sugars, amino acids, and certain drugs.

Optically Active Substances Cont’d

The extent of optical rotation depends on the concentration of the substance and the path length through which light passes.
Specific rotation (α) is a measure of the amount of rotation per unit length and concentration.
It is given by the equation: α = αobs/lc, where αobs is the observed rotation, l is the path length, and c is the concentration.

Optical Activity in Chiral Molecules

Enantiomers are pairs of molecules that are mirror images of each other but cannot be superimposed.
Enantiomers have the same physical and chemical properties except for their interaction with polarised light.
They rotate the plane of polarisation in opposite directions.

Specific Rotation and Enantiomeric Excess

The specific rotation of an optically active substance depends on its molecular structure.
Enantiomeric excess (ee) measures the ratio of one enantiomer to the other in a sample.
It is given by the equation: ee = (αobs x 100)/(αmax x l x c), where αmax is the specific rotation of pure enantiomer.

Optical Activity in Aqueous Solutions

Optically active substances can be dissolved in water and their rotation measured using a polarimeter.
This is particularly useful for determining the purity and concentration of chiral drugs and chemicals.

Optical Activity and Stereochemistry

The study of optical activity is closely related to the field of stereochemistry, which examines the spatial arrangement of atoms in molecules.
Different stereoisomers, such as cis-trans isomers and optical isomers, exhibit different optical activity.

Circular Dichroism

Circular dichroism (CD) is a spectroscopic technique that measures the difference in absorption of left-handed circularly polarised light and right-handed circularly polarised light.
CD spectra provide valuable information about the secondary structure of proteins and the chirality of molecules.

Polarisation and Wave Interference

Polarisation plays a crucial role in wave interference phenomena.
When two or more polarised waves interfere, their amplitudes and phases determine the resulting intensity and polarisation of the resulting wave.

Summary and Conclusion

Polarisation of light involves the alignment of electric field vectors in a particular direction.
It can be achieved through various processes such as reflection, scattering, transmission through polarisers, and double refraction.
Understanding polarisation is essential for applications in photography, optical instruments, communication, and scientific research.
The study of optical activity provides insights into the structure and properties of chiral molecules.
CD spectroscopy and polarisation play important roles in the fields of chemistry, biology, and material science.
Mastery of these concepts will enable students to better understand the behavior of light and its applications in various fields.