Darkness

Darkness is the condition resulting from a lack of illumination, or an absence of visible light. Under low light conditions, human vision becomes monochrome, a phenomenon termed scotopic vision. The lack of color comes from inactivity of cone cells, the type of photoreceptor cell on the retina that is responsible for the perception of color, but less sensitive to light than the rod cells. Scotopic vision takes place when luminance levels drops below the sensitivity threshold of cone cells; the other two type - mesopic and photopic vision - refers to vision under intermediate and normal lighting conditions. The emotional response to darkness has led to metaphorical usages of the term in many cultures, often to convey feelings of unhappiness or foreboding. "Darkness" may also refer to night, which occurs when the Sun is more than 18° below the horizon.