Light rays and ultraviolet and infrared radiation pass through the Earth's atmosphere and play a vital role in making organic life possible. The visible spectrum – the small portion of the electromagnetic spectrum to which the human eye is sensitive – extends from 380 nanometres (violet light) to 780 nm (red light).

Isaac Newton discovered that white sunlight contains five colours: violet, blue, green, yellow and red. His experiment was as simple as it was ingenious: he directed a focused beam of light onto a glass prism and projected the rays emerging from it onto a white surface, thus rendering the coloured light spectrum visible. It corresponds to the colours of the rainbow.

In a second experiment, Newton directed the coloured rays onto a second prism and found that the light that emerged from that was white. He had found the proof that white sunlight is the sum of all the colours of the spectrum.

Each spectral colour has a specific wavelength and the colour spectrum is continuous – from blue-violet to orange.

Spectral colours for accurate colour perception

Colours and coloured objects only appear coloured when the relevant colours are present in the spectrum of the light source. This is the case with the sun, for example, as well as with incandescent lamps and fluorescent lamps with very good colour rendering properties. Low-pressure sodium vapour lamps, on the other hand, lack the spectral colours red, blue and green, so objects that are illuminated by these lamps appear bathed in monochromatic yellow light.