The electromagnetic radiation theory of light implies that light consists of electric and magnetic components which vibrate at right angles to the direction of propagation.
In optical mineralogy only the electric component, referred to as the electric vector, is considered and is referred to as the vibration direction of the light ray.
The vibration direction of the electric vector is perpendicular to the direction in which the light is propagating.
The behaviour of light within minerals results from the interaction of the electric vector of the light ray with the electric character of the mineral, which is a reflection of the atoms and the chemical bonds within that minerals.
Light waves are described in terms of velocity, frequency and wavelength.
F = Frequency or number of wave crests per second which pass a reference points => cycles/second of Hertz (Hz).
For the purposes of optical mineralogy, F = constant, regardless of the material through which the light travels. If velocity changes, then the wavelength must change to maintain constant F.
Light does not consist of a single wave => infinite number of waves which travel together.