Shadow, a dark area within or next to an illuminated one. Shadows are caused by the blocking of light by an object. Large areas of shadow, as from trees, buildings, and other large bodies, are usually called shade. Because the heat-producing infrared rays from the sun are blocked as well as the visible light, it is usually cooler in the shade than in areas where the rays of the sun are not blocked. A full moon is bright enough to produce noticeable shadows, as are most types of illumination.

Most shadows consist of two parts, the umbra and the penumbra. In the umbra, all light from the source is blocked and an area of maximum darkness for the prevailing conditions results. In the penumbra, the opaque body blocks part of the light from the source, but not all of it. The penumbra, then, is an area of medium darkness. If the light comes from a point source, such as by shining a light through a pinhole in a piece of paper, the entire shadow is umbra; there is no penumbra.

Shadows play an important role in photography and art, where they may set a mood or strengthen a composition By using trigonometry, it is possible to determine the height of the objects that cast shadows, a technique used to measure the heights of mountains on the moon Shadows in aerial photographs help to bring out the three-dimensional character of the terrain. During a solar eclipse, the shadow of the moon falls on the earth Special studies of the sun are possible during such eclipses.