Human beings can only see light on the visible spectrum.
We see visible light when light waves reflect and bounce off particles. These particles can be atoms in objects, faces, dust particles, mountains, or even the tiny particles suspended in air or water. The particles scatter the light waves into our eyes, allowing us to see objects and colors.
Visible light waves vary in length, from violet at the short end of the spectrum, through indigo, blue, green, yellow, orange, and finally red at the long end.
Depending on what particles a light wave interacts with, its length and frequency is altered. Certain particles will absorb more of one type of wavelength and reflect another. This is what gives us color.