The eye, with its 130 million photorecpetor cells, accounts for 70% of all sensory input to the human brain and is one of the most fascinating and amazing products of evolution.
The eye is constantly bombarded by electromagnetic field quanta, called photons. Photons can have various wavelengths, and this gives the electromagnetic (EM) spectrum. The EM spectrum is huge. In principle the wavelengths can be zero to infinity. A wide range of radiation influences our daily life (radios, micro wave ovens, x-ray etc.). However, we are only able to see a tiny part of it. Our eyes can only see photons with wavelengths between 380-760 nanometres (nm), and this is normally referred to as visible. Our eyes are very sensitive; it takes no more than roughly 5-15 photons to trigger a visible flash. This corresponds to an energy of 4E-18 Joule [J].