darkness - choose a jigsaw puzzle to solve

Darkness, the polar opposite to brightness, is understood as a lack of illumination or an absence of visible light . Humans are unable to distinguish color in conditions of either high brightness or darkness. In conditions of insufficient light , perception is achromatic and ultimately, black . The emotional response to darkness has generated metaphorical usages of the term in many cultures. Complete darkness is when the sun is more than 18 degrees below the horizon .