The visible spectrum is comprised of all the colors that we're capable of seeing. What we are witnessing when we see pure white light, such as that which comes from the sun, is actually the effect of seeing all colours at the same time.
The visible spectrum, as we have already said, is a finite range of electromagnetic[EM] wavelengths within the EM spectrum. When we see one single colour in its purest form, let's say red, we are actually witnessing EM energy at one single wavelength. Every colour corresponds to a different wavelength of EM energy. Conversely, we perceive each and every separate wavelength within the visible band of the EM spectrum as a distinct colour.
Consider the effect of light passing through a crystal. Lay the crystal down on a piece of white paper under white light as the light passes through the structure of the crystal the different wavelengths succumb to the crystals refractive properties at different points in a sequential order. The" white light" effect is neutralized as the crystal presents us with the visible spectrum now displayed against the paper in perfect order [re: cover of Pink Floyd's Dark Side of the Moon].
