Waves and Light: Polarisation of Waves

Understanding Polarisation

Polarisation refers to the process in which transverse waves oscillate in one specific direction only, instead of multiple directions.
This phenomenon is particularly relevant to light and electromagnetic waves as they are transverse waves.
Polarisation is used across a variety of practical applications, including polarised sunglasses and LCD screens.

Polarising filters or polarisers have a microscopic structure that absorbs all orientations of light wave except for one.
These filters can convert unpolarised light into polarised light.
When a beam of unpolarised light passes through a polarising filter, the emerging light is plane polarised with the direction of oscillation parallel to the polarising direction of the filter.

Malus’ law states that the intensity (I) of plane-polarised light after passing through a second polarising filter is proportional to the square of the cosine of the angle (θ) between the transmission axes of the filters.
This can be mathematically represented as (I = I_0 \cos^2θ), where (I_0) is the initial intensity of the light.

Unpolarised light consists of waves with electric field vectors in random directions within a plane perpendicular to the direction of propagation.
If unpolarised light passes through a polarising filter, it emerges as plane-polarised light and reduces to half of its original intensity.
Plane-polarised light consists of waves wherein the field vectors oscillate in one single direction within a plane perpendicular to the direction of propagation.

Light can also become plane polarised by reflection from a non-metallic surface.
This is known as Brewster’s law and it states that the angle of incidence that will produce plane-polarised light is given by (tanθ = n), where (n) is the refractive index of the material.
At this angle, the reflected light is completely plane polarised.