shadow

Shadow is the region where light from a source is obstructed by an object. It is cast on surfaces opposite the light source and resembles the object's outline, though it can be distorted by the surface shape and light angle. The sharpness and size of a shadow depend on the size and distance of the light source, the distance between the object and the surface, and the surface properties. Point light sources produce well-defined shadows, while extended, diffuse sources create softer edges and a penumbra, a partially shaded border around the darker core. When the light source is blocked entirely, the fully shaded area is called the umbra; the partially shaded outer region is the penumbra.

In astronomy, shadows produced by celestial bodies create eclipses: the Moon's shadow on Earth causes solar eclipses; Earth's shadow on the Moon causes lunar eclipses.

In culture and art, shadows are used for mood, contrast, and form; in photography and cinema they help define depth. The term shadow is also used in psychology to denote unconscious aspects of the personality, a concept associated with Jung.

Sundials and shadow measurements have been used historically to tell time and gauge solar position.