The reason that the human eye can see the spectrum is because those specific wavelengths stimulate the retina in the human eye. The spectrum is arranged
in the order red, orange, yellow, green, blue, indigo, and violet according to the different wavelengths.
The light in the region with the longest wavelengths is seen as red, and the light in the region with the shortest wavelengths is seen as violet.
The light region which the human eye can see is called the visible light region.
If we move beyond the visible light region toward longer wavelengths, we enter the infrared region; if we move toward shorter wavelengths, we enter
the ultraviolet region. Both of these regions cannot be seen by the human eye.