Polarisation: Light's Hidden Order
Waves and Their Vibrations
To understand polarisation, we must first understand waves. Imagine a rope. If you shake one end up and down, you create a wave that travels along the rope. The rope moves perpendicular (at a right angle) to the direction the wave is traveling. This is called a transverse wave. Light is a type of transverse wave, but instead of a rope, it's the electric and magnetic fields that are vibrating.
Now, imagine shaking that rope in every possible direction—up-down, left-right, and everything in between. This is what ordinary, unpolarised light is like. Its vibrations are occurring in all possible planes perpendicular to its direction of motion. Polarisation is the process of taming this chaotic vibration, forcing the wave to oscillate in only one specific plane.
The amount of light that passes through a polariser is governed by Malus's Law. If $I_0$ is the initial intensity of polarised light, and $I$ is the intensity after it passes through a second polariser (called an analyzer), then:
$I = I_0 \cos^2\theta$
Here, $\theta$ (the Greek letter theta) is the angle between the light's polarisation direction and the axis of the analyzer. When $\theta = 0^\circ$, all light passes through ($\cos 0 = 1$). When $\theta = 90^\circ$, no light passes through ($\cos 90 = 0$).
How Do We Polarise Light?
There are several common methods to transform unpolarised light into polarised light. Each method acts like a filter with microscopic slits, only allowing vibrations parallel to the slits to pass through.
| Method | How It Works | Common Example |
|---|---|---|
| Absorption (Polaroid Filter) | A material with long-chain molecules acts like a microscopic picket fence. It absorbs light vibrating in one direction and transmits light vibrating in the perpendicular direction. | Polarising sunglasses and the filters used in photography. |
| Reflection | When light reflects off a non-metallic surface like water or glass, the reflected light becomes partially polarised. At a specific angle (Brewster's Angle[1]), it is completely polarised. | Glare from a horizontal road or a lake. |
| Scattering | As sunlight passes through the atmosphere, air molecules scatter the light. The scattered light we see from the sky is partially polarised. | The blue sky itself is a source of polarised light. |
| Birefringence (Double Refraction) | Some crystals, like calcite, have two different refractive indices. They split a single light ray into two rays that are polarised at right angles to each other. | Used in liquid crystal displays (LCDs) and in scientific instruments. |
Polarisation in Action: From Sunglasses to 3D Movies
Polarisation isn't just a laboratory curiosity; it's a principle that powers many technologies we use every day.
Polarising Sunglasses: These are the most common example. Glare from a road or water is mostly horizontally polarised light. The lenses in polarising sunglasses are vertically polarised. Using Malus's Law, when the horizontally polarised glare meets the vertically oriented filter in the glasses ($\theta = 90^\circ$), it is blocked. This dramatically reduces glare, making it easier and safer to see.
Liquid Crystal Displays (LCDs): Your computer monitor, TV, and smartphone screen likely use polarisation. An LCD screen has a backlight that passes through a polarising filter. The light then travels through a layer of liquid crystals, which can be electronically controlled to rotate the light's polarisation plane. Finally, it passes through a second polarising filter (the analyzer). By precisely controlling which pixels rotate the light, the analyzer blocks or transmits light to create the dark and bright images you see.
3D Cinema: Modern 3D movies use polarisation to create the illusion of depth. The movie is filmed with two cameras, simulating left and right eyes. The projectors then send these two images onto the screen, each polarised at a different angle (e.g., one at $0^\circ$ and one at $90^\circ$). The 3D glasses you wear have corresponding lenses—the left lens has a polariser that only allows the $0^\circ$ image through, and the right lens only allows the $90^\circ$ image. Your brain merges these two slightly different images to perceive a three-dimensional scene.
Common Mistakes and Important Questions
Can sound waves be polarised?
If I hold two polarising filters and rotate them, why does it go dark when they are crossed?
Is all light polarised?
Polarisation is a fascinating and tangible property of light that reveals the hidden, directional nature of its wave vibrations. From the simple act of putting on sunglasses to cut glare to the complex technology behind our digital screens and 3D entertainment, the principles of polarisation are deeply woven into our daily lives. By understanding that light can be filtered to vibrate in a single plane, we unlock a deeper appreciation for the behavior of waves and the ingenious ways we can harness them. It is a perfect example of a fundamental scientific concept with powerful and widespread practical applications.
Footnote
[1] Brewster's Angle: The specific angle of incidence at which light with a particular polarisation is perfectly transmitted through a transparent surface, with no reflection. The reflected light at this angle is completely polarised. It is named after the Scottish physicist Sir David Brewster.
[2] LCD (Liquid Crystal Display): A flat-panel display that uses the light-modulating properties of liquid crystals combined with polarisers. Liquid crystals do not emit light directly but use a backlight to produce images in color or monochrome.
[3] Transverse Wave: A wave that oscillates perpendicular to the direction in which the wave is traveling. Examples include light waves and waves on a string.
[4] Longitudinal Wave: A wave that oscillates parallel to the direction in which the wave is traveling. The primary example is a sound wave.