1. What polarisation methods do
Polarisation methods restrict the direction in which the electric field of light vibrates. Since natural light vibrates in many random directions, these methods help convert it into plane-polarised light, where vibrations occur in only one direction.
2. Polarisation by transmission (Polaroid filters)
A polaroid sheet contains long-chain molecules aligned in one direction. These molecules absorb electric field vibrations parallel to their length and allow perpendicular vibrations to pass. This produces linearly polarised light.
2.1. How polaroids work
The sheet acts like a molecular grid. If the incoming vibration is along the absorbing direction, it gets blocked. If it is perpendicular, it passes through.
Polaroids are widely used in sunglasses, camera filters, and LCD screens.
3. Polarisation by reflection
When unpolarised light strikes a surface like water, glass, or a road, the reflected beam becomes partly polarised. At a particular angle called Brewster’s angle, the reflected light becomes completely plane-polarised.
3.1. Brewster’s law
Brewster’s angle \(i_B\) satisfies:
\( \tan i_B = \mu \)
where \(\mu\) is the refractive index of the surface.
3.2. Daily-life example
The glare from water or a shiny road surface is mostly horizontally polarised. Polarised sunglasses block this horizontal glare, making the view clearer.
4. Polarisation by refraction (Double refraction)
Certain crystals like calcite and quartz split a beam of unpolarised light into two separate beams — the ordinary ray (O-ray) and the extraordinary ray (E-ray). These two rays are polarised in mutually perpendicular directions.
4.1. Why double refraction happens
These crystals have an internal structure that makes the speed of light depend on the direction of vibration. This causes the original beam to break into two polarised components.
5. Polarisation by scattering
When sunlight interacts with atmospheric molecules, part of it becomes polarised. Scattered light at 90° to the Sun’s direction is strongly polarised. This effect helps explain why the sky looks different when viewed through a polarising filter.
6. Polarisation by absorption
In some materials, certain vibration directions are absorbed more strongly than others. The transmitted light ends up vibrating mainly along the allowed direction. Polaroid sheets use this principle.
7. Polarisation by reflection from metallic surfaces
Reflected light from metals is usually not plane-polarised but is often elliptically polarised. This shows that different materials can affect the incident vibrations differently.
8. Rotatory polarisation (Optic rotation)
Some substances like sugar solutions and quartz rotate the plane of polarisation as light passes through them. The amount of rotation depends on the concentration and thickness of the material.
9. Uses of different polarisation methods
Different methods of polarisation are used in optics, communication, and everyday technology:
- Polaroids → sunglasses, LCD screens, camera lenses
- Double refraction → optical instruments, polarimeters
- Reflection → glare reduction
- Scattering → understanding sky polarisation
- Optic rotation → measuring sugar concentration, chemical analysis