Light is a very small portion of the electromagnetic spectrum perceived by human eyes. Such portion is called visible spectrum and the wavelengths that define it range more or less from 380 and 750 nm (nanometers).
Our eyes can see and distinguish colors in the visible spectrum thanks to cone cells, placed in the fovea. There are around 6 millions cone cells, divided in three types: those that define red colors (maximum sensitivity at 565 nm more or less), green colors (maximum sensitivity at 530 nm) and blue colors (maximum sensitivity around 435 nm). All of this happens when light has a level high enough to make them work (photopic vision), like daylight or good artificial lighting. In our retina there are also around 120 millions retinal rods that allow us to see at night (scotopic vision) and to define shape in grey monochromatic tones.
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