The Entire Electromagnetic Spectrum is defined by the wavelength of the
light (which is the same as saying the frequency). For instance, RED light
has a longer wavelength than BLUE light. When it passes through a prism,
it breaks up the light into its individual components, based on the
wavelength.
However, the OPTICAL represents only a very, very small portion of the entire
Electromagnetic Spectrum, which is shown in its entirety below: