Introduction

Light waves exhibit a fascinating property called polarisation, which provides compelling evidence for light's transverse wave nature. This fundamental concept helps us understand how light interacts with materials and forms the basis for many modern technologies.

The Nature of Light Waves

Light consists of electromagnetic waves where electric and magnetic fields oscillate perpendicular to each other and to the direction of wave propagation. The direction in which the electric field oscillates defines the wave's polarisation axis.

Light can exist in two polarisation states:

Polarising Filters

Polarising filters (or polaroids) are special materials that can convert unpolarised light into polarised light. These filters contain molecules aligned in a specific direction, creating what's known as a transmission axis. When light passes through the filter:

The Picket Fence Analogy

A useful way to understand polarising filters is to imagine a picket fence. The vertical slats represent the filter's transmission axis:

Malus's Law

When polarised light passes through a polarising filter, the intensity of transmitted light follows Malus's Law:

$I = I_0\cos^2\theta$

Where:

Polarisation of Unpolarised Light

When unpolarised light passes through a polarising filter, its intensity is always reduced by 50%, regardless of the filter's orientation. This occurs because unpolarised light can be considered to have its intensity equally distributed across all possible angles. Mathematically:

$\text{Average value of }\cos^2\theta = 0.5$ for angles from 0° to 360°

Evidence for the Wave Nature of Light

Polarisation phenomena provide strong evidence for the transverse wave nature of light:

Applications

Understanding light polarisation has led to numerous practical applications: