When all the frequencies of visible light are combined, the resulting light is perceived as white by the human eye. White light contains all frequencies of light at roughly equal intensities. The process of splitting white light into its component colors is called dispersion.
Dispersion works because light of different wavelengths is deflected through transparent solids at different angles. The particular angle at which the light is deflected is called the index of refraction. Because the index of refraction is slightly different for each color, even as they travel through the same medium, white light can be split, revealing the full visible spectrum. Isaac Newton's experiments using a prism to refract and reconstitute sunlight proved that white light was made up of many different colors.