Polarisation

What is Polarisation?

When astronomers observe the Universe, we usually measure how much light an object emits (photometry) or how that light is distributed with wavelength (spectroscopy). These techniques have transformed our understanding of stars, galaxies, planets, and the cosmos.

But light has another equally fundamental property: polarisation.

Like intensity and wavelength, polarisation is an intrinsic property of light. While intensity tells us how much light is present and wavelength tells us its colour, polarisation describes the orientation and behaviour of the light’s oscillating electric field. It often contains information that is completely invisible to imaging and spectroscopy, revealing magnetic fields, scattering geometries, dust grains, and three-dimensional structures that would otherwise remain hidden.

Light is an electromagnetic wave consisting of oscillating electric and magnetic fields that are perpendicular both to each other and to the direction of propagation. Since astronomical detectors respond primarily to the electric field, polarimetry measures the orientation and properties of this electric field.

For most astronomical sources, light is produced by an enormous number of independent emitters, each radiating with a random electric field orientation. The resulting light is therefore unpolarised.

However, when light is scattered, reflected, emitted in strong magnetic fields, or passes through aligned interstellar dust grains, certain orientations of the electric field become preferred. The light then becomes polarised.

By measuring these often tiny changes in the orientation of the electric field, astronomers can infer physical processes that are otherwise inaccessible through imaging or spectroscopy.