The electromagnetic spectrum and the visible range

A vast spectrum

Visible light is only a tiny part of the electromagnetic spectrum, which encompasses all electromagnetic waves classified in order of increasing wavelength: gamma rays, X-rays, ultraviolet (UV) rays, visible light, infrared (IR), microwaves and radio waves.

The visible spectrum

The human eye can only perceive wavelengths (in a vacuum) ranging from approximately 400 nm (violet) to 800 nm (red).

Colour	Wavelength (nm)
Violet	400–450
Blue	450–490
Green	490–560
Yellow	560–590
Orange	590–620
Red	620–800

White light (such as that from the Sun) is a combination of all these colours: this is demonstrated by its decomposition through a prism.

Dispersion

In a transparent medium such as glass, the refractive index n depends slightly on the wavelength (n increases as λ decreases). Violet is therefore deflected further than red during refraction: this is the phenomenon of dispersion, which is responsible for rainbows and the decomposition of light by a prism.

Common misconception

Do not confuse ‘white light’ (a mixture of all colours) with ‘monochromatic light’ (a single wavelength, a single frequency). A laser is a monochromatic source; a standard light bulb is polychromatic.