What determines a wave's direction of polarisation?

A wave's direction of polarisation is determined by the orientation of the electric field vector.

In more detail, electromagnetic waves, such as light, are transverse waves that consist of oscillating electric and magnetic fields. These fields are perpendicular to each other and to the direction of propagation of the wave. The direction of polarisation of a wave is defined by the direction in which the electric field vector oscillates.

For instance, if the electric field vector oscillates vertically, the wave is said to be vertically polarised. If it oscillates horizontally, the wave is horizontally polarised. It's also possible for the electric field to oscillate at any angle, or even to rotate, leading to other types of polarisation such as circular or elliptical polarisation.

Polarisation can be naturally occurring, such as in the case of light reflected off certain surfaces, or it can be induced artificially using polarising filters. These filters work by only allowing waves with a specific direction of polarisation to pass through, effectively blocking all others.

In the context of wave optics, understanding polarisation is crucial. It helps in the study of various optical phenomena like reflection, refraction, interference, and diffraction. Moreover, polarisation has numerous practical applications, from the creation of 3D movies and polarised sunglasses to advanced scientific and technological uses in fields like telecommunications, microscopy, and quantum computing.

In summary, the direction of polarisation of a wave is determined by the orientation of its electric field vector. This concept is a fundamental aspect of wave optics and has wide-ranging implications and applications.