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:



Discovery of Infrared Radiation
Discovery of Ultraviolet Radiation