According to the electromagnetic theory of light, the state of polarisation of an electromagnetic wave is defined by

the electric field only
the magnetic field only
both the electric and magnetic fields
the direction of propagation

Correct answer: the electric field only

Solution

By convention, the direction of polarisation of an electromagnetic wave is taken as the direction of its electric field vector, because the electric field governs most light-matter interactions (e.g. with charges in detectors).

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