Polarisation

POLARIZATION OF LIGHT WAVES

Definition of Polarization

  • Confinement of light waves in a particular direction is called polarization of light waves.

  • Establishes that light waves are transverse in nature.

Wave Motion

  • Longitudinal Wave: Particles of the medium move to and fro in the direction of propagation (e.g., sound waves).

  • Transverse Wave: Particles of the medium vibrate up and down perpendicular to the direction of propagation (e.g., ripples on water).

Mechanical Experiment to Illustrate Polarization

  • A string passing through two vertical slits (S1 and S2) cut in cardboard held by two persons demonstrates wave polarization.

Effects of Polarization

  • When ordinary light ray hits tourmaline crystal plates A and B cut parallel to their crystallographic axes, emergent light shows variation as B is rotated.

  • Intensity is maximum when the axis of B is parallel to A and minimum at right angles.

  • This indicates light vibrations are confined to a single line in a plane perpendicular to propagation (plane polarized or linearly polarized light).

Electromagnetic Theory of Light

  • Light waves consist of electric and magnetic vectors vibrating in mutually perpendicular planes, both perpendicular to the direction of propagation.

  • Electric vector acts as the light vector; in plane polarized light, it vibrates along a fixed line perpendicular to propagation.

Plane of Vibration and Plane of Polarization

  • Plane of Vibration: The plane containing the direction of vibration, perpendicular to propagation.

  • Plane of Polarization: Perpendicular to the plane of vibration, contains no vibrations.

Pictorial Representation of Light Vibrations

  • Shows representations of unpolarized and polarized light with respective vibrations in reference to the plane of paper.

Types of Polarization

  • Plane Polarized Light: Electric vector vibrates in a fixed straight line perpendicular to the direction of light propagation.

  • Circularly Polarized Light: Resultant light vector rotates with constant magnitude in a plane perpendicular to propagation.

  • Elliptically Polarized Light: Resultant vector rotates in an elliptical pattern when two plane polarized lights are superimposed.

Methods to Produce Plane Polarized Light

  1. Polarization by reflection (e.g., Biot Polariscope)

  2. Polarization by refraction (e.g., Piles of Plates method)

  3. Polarization by double refraction (e.g., Nicol Prism)

  4. Polarization by scattering

  5. Polarization by selective absorption by crystals (e.g., Polaroids)

Polarization by Reflection

  • At a 90° angle between reflected and refracted rays, reflected light is linearly polarized. Direction of polarization is parallel to the interface plane.

Brewster's Law

  • The tangent of the polarizing angle equals the refractive index of the reflecting material.

  • Mathematically represented.

Proof of Brewster’s Law

  • Proof involving unpolarized light at the polarizing angle on a plane surface of a transparent medium.

  • Involves application of Snell's law.

Polarization by Refraction

  • Ordinary light refracted through a transparent medium is partially polarized. Complete polarization occurs via multiple reflections in piles of glass plates separated by air gaps.

Malus Law

  • Intensity of emergent light varies as the square of the cosine of the angle between the polarizer and analyzer transmission planes.

Double Refraction

  • Doubly Refracting Crystals: Split light into two rays—the ordinary ray (o-ray) obeys laws of refraction, while the extraordinary ray (e-ray) does not.

  • Types: Uniaxial (single optic axis) and Biaxial (two optic axes).

Applications of Polarization

  • Instruments for measuring specific rotation and concentration of solutions using polarimeters.

Parameters for Polarimetric Measurements

  • Specific Rotation defined for solids and solutions.

  • Rotational measurement in degrees per decimeter of solution concentration.

Summary of Key Concepts

  • Polarization involves the behavior and characteristics of light waves, fundamentally affecting their propagation and interaction with materials.

  • Understanding the types of polarization, methods to produce polarized light, relevant laws, and mathematical concepts is essential for applications in physics and engineering.