If sunlight passes through a prism, a band of colours like a rainbow is produced. This was discovered by Isaac Newton, who also formulated the theory of universal gravitation. The band of colours is called the colour spectrum. The splitting of light in this way is called "dispersion". A rainbow is a colour spectrum produced when sunlight passes through prisms consisting of water vapor (water droplets).

In terms of physics, light is a type of electromagnetic wave. The distance from trough to trough (or from peak to peak) of these waves is called the wavelength. The colours we perceive differ depending on the wavelength of the light we are seeing. The reason human beings can see the colour spectrum is that certain specific wavelengths of light stimulate the retinas of our eyes, causing us to perceive colours. The colours are ordered in the spectrum in the sequence red, orange, yellow, green, blue, indigo, violet. This phenomenon results from the fact that each of these colours has a different wavelength. We perceive the light with the longest wavelengths as red and that with the shortest wavelengths as violet. The range of wavelengths the human eye is capable of perceiving is referred to as "visible light".

Light with wavelengths longer than the longest in the range of visible light is said to be in the infrared range. Conversely, light with wavelengths shorter than the shortest in the range of visible light is said to be in the ultraviolet range. The human eye cannot see either infrared or ultraviolet light. We can, however, perceive an enormous range of different colours, depending on the strength or weakness of the different wavelengths(the proportions in which they are mixed together).