1. What polarisation means
Light is a transverse electromagnetic wave, which means its electric field vibrates perpendicular to the direction of travel. In unpolarised light, these vibrations occur in all possible directions perpendicular to the path of the wave.
Polarisation is the process of restricting these vibrations to a single plane. After polarisation, the electric field vibrates in only one direction.
2. Unpolarised vs polarised light
Most natural light sources (Sun, bulbs, candles) emit unpolarised light. This means the electric field is randomly oriented.
2.1. Unpolarised light
Electric field vibrates in many directions. It looks like a mix of waves pointing differently.
2.2. Polarised light
Electric field vibrates only in one specific direction — the wave becomes more orderly. A polaroid sheet or reflection from certain surfaces can produce polarised light.
3. How polarisation occurs
Polarisation is possible because light is a transverse wave. Only transverse waves have vibrations perpendicular to the direction of travel that can be filtered.
3.1. 1. Polarisation by transmission (Polaroid filters)
A polaroid sheet has long-chain molecules aligned parallel to each other. These molecules absorb electric field vibrations in one direction and allow only the perpendicular direction to pass. As a result, the output light is polarised.
3.2. 2. Polarisation by reflection
When light reflects off a shiny surface like water, roads, or glass at a special angle (Brewster’s angle), the reflected beam becomes partly or fully polarised.
3.3. 3. Polarisation by scattering
Atmospheric particles scatter sunlight. During scattering, light vibrating in certain directions is favoured, causing partial polarisation. This is why the sky appears polarised when seen through a polarising filter.
4. Polarisation by reflection and Brewster’s angle
At a particular angle called Brewster’s angle, the reflected ray becomes fully plane polarised.
Brewster’s law gives the relationship:
\( \tan i_B = \mu \)
where:
- \(i_B\) = Brewster’s angle
- \(\mu\) = refractive index of the reflecting surface
5. Malus’s law for polarised light
Malus’s law explains how the intensity of polarised light changes when it passes through an analyser (a second polaroid).
If \(I_0\) is the initial intensity and \(\theta\) is the angle between the polariser and analyser, then:
\( I = I_0 \cos^2 \theta \)
Maximum intensity occurs at \(\theta = 0^\circ\), and zero intensity at \(\theta = 90^\circ\).
6. Why polarisation is useful
Since polarisation filters out light vibrating in unwanted directions, it has many everyday uses:
- Reducing glare from roads, water, and car windshields
- Improving contrast in sunglasses
- Stress analysis in transparent materials
- Liquid crystal displays (LCD screens)
- 3D movie glasses (each eye receives differently polarised light)
7. Example: Why polarised sunglasses reduce glare
Reflected light from flat surfaces is mostly polarised horizontally. Polarised sunglasses have a vertical transmission axis. They block horizontal vibrations and allow vertical ones to pass.
This reduces glare sharply and makes images clearer and more comfortable to view.
8. Polarisation proves the transverse nature of light
Only transverse waves can be polarised. Since light can undergo polarisation, it must be a transverse wave. This is one of the strongest pieces of evidence for the wave model of light.