Light is a transverse wave, meaning oscillations are perpendicular to the direction of travel.

→ Only transverse waves undergo polarisation. The effects of polarisation can be seen in other transverse waves, such as microwaves, but not longitudinal waves, like sound waves.

Light which is not polarised has electric field oscillations in every plane perpendicular to its travel (these can be resolved into oscillations in the horizontal and vertical planes). Polarisation causes light to oscillate in one plane only.

→ Polarisation is used in polarising sunglasses to reduce the glare of reflected light on a water surface.

Polarisation can be caused by filters called polarisers and analysers, e.g. a polaroid can plane polarise light.

A polariser polarises light to one plane of oscillation, e.g. the $y$-axis. If a polariser is aligned with the plane of oscillation of polarised light, then the light will pass through. A second polariser placed on the first is called an analyser — if it is aligned with the plane of oscillation of the polarised light, then maximum light will pass through, if it is perpendicular to the plane of oscillation of the polarised light then no light will pass through, between these two angles some light will pass through.

Polarisation can also be caused by reflection. When a ray of unpolarised light is incident on the surface of an insulator at Brewster’s angle the reflected ray becomes plane-polarised.

→ Brewster’s angle, $i_p$, is the angle of incidence which causes the reflected ray to be completely plane polarised or the angle of incidence which causes the angle between the reflected ray and the refracted ray to be $90\degree$.

→ $i_p+90\degree + r = 180\degree$

$i_p+r=90\degree$

→ $n=\dfrac{sin\theta_1}{sin\theta_2}$ where $n=n_{glass}$ and $\theta_1=i_p$, $\theta_2=r$

$n=\dfrac{sini_p}{sinr}$ and $r=90\degree-i_p$