Generally, light is composed of a mixture of wavelengths: when these wavelengths are separated into components (using for example a prism) light forms a spectrum, which can be defined as the intensity of radiation as a function of wavelength or frequency. Visible light (the white light) is just a part of the electro-magnetic spectrum, ranging from the smallest visible wavelength for violet (about 35 millionths of a centimeter), to 75 millionths of a centimeter for red. (click here to know more about radiation, waves and their wavelength and frequencies). Higher frequencies (corresponding to shorter wavelengths) are called ultraviolet and X rays. Lower frequencies (which are at longer wavelengths) are called infrared, and radiowaves.

The rainbow is a natural spectrum, produced by meteorological phenomenon. A similar effect can be produced by passing sunlight through a glass prism. The first correct explanation of the phenomenon was advanced in 1666 by the English mathematician and physicist Sir Isaac Newton.