This section describes the nature of light, how it is perceived by
the eye and how it is measured.

1. The Nature of Light

Light is one of the forms of energy known as electromagnetic radiation, which also includes heat, radio waves and X-rays.  Electromagnetic radiation travels outwards from its source in a waveform, like ripples in a pond.  Electromagnetic waves travel in space at approximately 300,000 kilometres per second.  This is commonly known as the speed of light, but it is the same for all electromagnetic waves.

Below: the electromagnetic spectrum.