Rainbows and spectra

The visible spectrum

A spectrum is produced by splitting white light into the different wavelengths which it contains.

These three primaries relate to coloured light and are called the Additive Primaries. Another set of three colours, the Subtractive Primaries, is used when dyes or pigments are applied to substrates such as textiles or paper. We see subtractive colours when light is reflected from surfaces.

We can create our own rainbows with a source of white light and a prism. If a narrow beam of white light is projected on to a prism, the light beam is refracted on entry and exit. The different wavelengths that together make up white light are bent to differing extents - the longer wavelengths like red bend less than the shorter ones like violet - and a spectrum of colour is produced. The colours merge into each other because there is a continuous range of wavelengths over the spectrum; they do not occur in separate blocks.