polarisation

Polarisation is a phenomenon unique to transverse waves where oscillations occur in a single plane perpendicular to the direction of energy propagation. Light, being an electromagnetic wave with oscillating electric and magnetic fields perpendicular to each other and to the direction of travel, can be polarised.

Unpolarised light contains waves vibrating in all possible planes perpendicular to the direction of propagation. Natural sunlight and light from incandescent bulbs are unpolarised. Plane-polarised light (or linearly polarised light) contains waves oscillating in only one plane.

Key Terms:

• Polariser: A filter that transmits light oscillating in only one plane (the transmission axis) while absorbing oscillations perpendicular to it
• Analyser: A second polariser used to detect or analyse already polarised light
• Plane of polarisation: The plane containing the direction of propagation and the direction of electric field oscillation

Malus's Law describes intensity transmission through crossed polarisers:

I = I₀ cos²θ

Where:

• I = transmitted intensity
• I₀ = incident polarised light intensity
• θ = angle between polariser and analyser transmission axes

Critical Understanding: When θ = 0° (parallel), maximum transmission occurs (I = I₀). When θ = 90° (crossed/perpendicular), no light transmits (I = 0). At θ = 45°, exactly half the intensity passes through.

Mnemonic: "LOTUS" - Light Oscillates Transversely, Unpolarised Sources. Only transverse waves can be polarised; longitudinal waves (like sound) cannot.

Polarisation methods: Reflection (Brewster's angle), scattering, and selective absorption (Polaroid filters).

Understanding Polarisation Through Analogy: Imagine pushing a rope threaded through vertical railings. You can only create waves that move up-down (vertically); side-to-side waves are blocked. The railings act like a polariser, allowing oscillations in one plane only.

Real-World Applications:

1. Polaroid Sunglasses: When sunlight reflects off horizontal surfaces (water, roads, snow), it becomes partially plane-polarised in the horizontal plane, causing intense glare. Polaroid sunglasses contain vertical polarisers that block horizontally polarised light, reducing glare dramatically. Fishermen use polarised glasses to see beneath water surfaces by eliminating surface reflections.

2. Photography & Cinema: Polarising filters on cameras reduce reflections from glass and water, enhance sky blueness, and improve colour saturation. 3D cinema technology uses two projectors with perpendicular polarising filters; viewers wear glasses with corresponding polarisers for each eye, creating stereoscopic depth perception.

3. LCD Screens: Liquid Crystal Displays exploit polarisation. Two perpendicular polarisers sandwich liquid crystals. Applied voltage rotates light polarisation, controlling pixel brightness. Your phone screen uses this principle!

4. Stress Analysis: Engineers use photoelasticity where transparent plastic models under mechanical stress rotate polarised light. Viewed between crossed polarisers, stress patterns appear as colourful fringes, revealing structural weaknesses.

5. Nature: Honeybees navigate using polarised skylight patterns. Light scattered by the atmosphere becomes partially polarised, creating a celestial polarisation map bees use as a compass, even on cloudy days.

Key Insight: Polarisation proves light is a transverse wave—this was historically crucial evidence for the electromagnetic wave theory of light, distinguishing it from sound (longitudinal).