Radiant energy sends out waves in the same way that a rock makes circular waves when thrown into a pond. The many kinds of radiant energy create waves of varying lengths. Some have short waves, while others have very long waves.
Small waves can be shorter than a billionth of a meter.
Very long waves, like radio waves, can be as long as a kilometer. The range of these waves is called the Electromagnetic Energy Spectrum. The human eye cannot see all radiant energy waves. It cannot see radio waves, for example. There is a range that it can see.