Polarisation

Polarisation is a property of transverse waves, such as light, where the vibrations occur in a particular direction perpendicular to the direction of wave travel. In an unpolarised transverse wave, the vibrations occur in all directions perpendicular to the direction of propagation. When a wave is polarised, its vibrations are restricted to a single plane.

Longitudinal waves (e.g., sound waves) cannot be polarised, because their oscillations are parallel to the direction of propagation.

Polarisation of Light

Light from most sources is unpolarised. When unpolarised light passes through a polarising filter (also called a polaroid), only the component of light vibrating parallel to the filter’s transmission axis passes through. The transmitted light is plane-polarised.

If already plane-polarised light passes through a second polarising filter (analyser), the intensity of the transmitted light depends on the angle $\theta$ between the light’s plane of polarisation and the transmission axis of the analyser.

Malus’s Law

Malus’s law describes how the intensity of plane-polarised light changes as it passes through a polarising filter at an angle.

If $\theta = 0^\circ$, all the light passes through ($I = I_0$). If $\theta = 90^\circ$, no light passes through ($I = 0$).

Applications and Significance

• Polarisation is evidence that electromagnetic waves are transverse.
• Used in polaroid sunglasses to reduce glare (by blocking horizontally polarised light).
• Used in photography, LCD screens, and stress analysis in materials.

Summary

• Only transverse waves can be polarised.
• Polarising filters restrict vibrations to a single plane.
• Malus’s law relates transmitted intensity to the angle between the plane of polarisation and the filter’s axis.