Polarisation — Definition

Definition

Imagine a rope tied to a wall. If you shake the free end up and down, the waves travel along the rope, and the vibrations are only in the vertical plane. If you shake it side-to-side, the vibrations are only in the horizontal plane.

Now, imagine shaking the rope in all possible directions perpendicular to its length – up-down, side-to-side, diagonally – all at once. This is similar to 'unpolarised' light. Its electric field (and magnetic field) vibrations occur in all possible planes perpendicular to the direction the light is travelling.

Now, what if we put a narrow vertical slit in front of our rope? If you shake the rope vertically, the wave passes through. If you shake it horizontally, it gets blocked. If you shake it in all directions, only the vertical components of the vibrations can pass through the slit. The wave that emerges on the other side now only vibrates in the vertical plane. This process of restricting the vibrations of a wave to a single plane is called 'polarisation'.

For light, which is a transverse electromagnetic wave, the 'vibrations' we talk about are the oscillations of its electric field vector. Unpolarised light, like that from the sun or a bulb, has electric field vectors oscillating randomly in all directions perpendicular to the direction of propagation.

When light undergoes polarisation, these random oscillations are restricted. If they are restricted to a single plane, it's called 'plane-polarised' or 'linearly polarised' light. This can happen when light interacts with certain materials or surfaces.

Think of sunglasses that reduce glare. They often use polarising filters. Glare from a wet road or water surface is often partially polarised horizontally. The sunglasses have vertical polarising filters, which block the horizontally polarised glare, allowing only the vertically polarised light (or unpolarised light) to pass through, making vision clearer.

This simple example highlights the practical application of polarisation, transforming chaotic, multi-directional vibrations into an ordered, single-plane oscillation.