Visible light may be defined as any radiation capable to act upon the retina of the human eye causing a visual sensation.
Each simple radiation differs from others by its frequency, i.e. its wavelength.
The representation of a radiation by its wavelength is generally accepted, because the wavelength can be measured with great accuracy.
The graphical representation of a spectrum of electromagnetic radiation is also based on wavelengths.
The visible part of the spectrum spans the relatively narrow range between 380nm and 780nm.
These limits represent experimentally obtained averages, because, in reality, they may vary from one individual to another.