You can read this page because your eyes detect the light reflected from it.
Light consists of electromagnetic waves.
The different colors of light are electromagnetic waves of different lengths. Visible light, however, covers only a small part of the range of wavelengths in which electromagnetic waves can be produced. Radio waves are electromagnetic waves of much greater wavelength than those of light. For centuries, astronomers learned about the sky by studying the light coming from astronomical objects, first by simply looking at the objects, and later my making photographs. Many astronomical objects emit radio waves, but that fact wasn't discovered until 1932.