Visible light represents only a small fraction of the full electromagnetic spectrum - the energy emitted by the sun. A normal human eye is sensitive only to wavelengths within the approximate range 400 -720 nanometres (nm), although in some instances people are able to perceive wavelengths between 380 and 780 nm.
Maximum sensitivity is normally at about 560 nm, which is the region where green merges with yellow. Within this visible spectrum, particular wavelengths give rise to certain visual sensations. For example, the shorter wavelengths are seen as violet and blue, and the longer ones orange or red. Nevertheless it is important to understand that terms such as "blue light" are used for convenience only. Colour exists only in the mind.
