Dark

Darkness, the polar opposite to brightness, is understood to be the condition of a very small amount or even 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.

The perception of darkness differs from the mere absence of light due to the effects of after images on perception. In perceiving, the eye is active, and the part of the retina that is unstimulated produces a complementary afterimage.

In terms of physics, an object is said to be dark when it absorbs photons, causing it to appear dim compared to other objects. For example, matte black paint does not reflect much visible light and appears dark, whereas white paint reflects lots of light and appears bright. For more information see color. An object may appear dark, but it may be bright at a frequency that humans cannot perceive.

A dark area has limited light sources, making things hard to see. Exposure to alternating light and darkness (night and day) has caused several evolutionary adaptations to darkness. When a vertebrate, like a human, enters a dark area, its pupils dilate, allowing more light to enter the eye and improving night vision. Also, the light detecting cells in the human eye (rods and cones) will regenerate more unbleached rhodopsin when adapting to darkness.

One scientific measure of darkness is the Bortle Dark-Sky Scale, which indicates the night sky's and stars' brightness at a particular location, and the observability of celestial objects at that location. (See also: Sky brightness)

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