The phenomenon of splitting of visible light into its component colours is called dispersion. Dispersion of light is caused by the change of speed of light ray (resulting in angle of deviation) of each wavelength by a different amount. Note: White light comprises of seven visible colours (VIBGYOR) whereas black (or dark) signifies the absence of light rays.
Splitting of white light
White light is made up of ranges of colours of which seven are not seen in a separated pattern. The splitting up of white light into its constituent colours is called dispersion. In the rainbow you may not see all the seven colours. It is due to overlapping of colours. The degree of bending of the light's path depends on the angle that the incident beam of light makes with the surface, and on the ratio between the refractive indices of the two media (Snell's law) and results in splitting all the seven colours present in white light. With the help of a narrow beam of light, glass prism and arrangement of lenses it is possible to produce the band of seven colours using white light. This band is called spectrum. There are many types of spectrum. It is not necessary that the source of light should always be white to get a spectrum. Composite light which contains a range of 3 to 4 colours also produces a band of 3 to 4 colours. A glowing 40W tungsten filament bulb does not produce pure white light. It is a source of composite light.It will also produce a spectrum but the spectrum may not be the same as the spectrum of white light. The type of spectrum depends upon the nature of source of light.