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A shadow is a region where light from a light source is obstructed by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the light. == Point and non-point light sources == A point source of light casts only a simple shadow, called an "umbra". For a non-point or "extended" source of light, the shadow is divided into the umbra, penumbra and antumbra. The wider the light source, the more blurred the shadow becomes. If two penumbras overlap, the shadows appear to attract and merge. This is known as the Shadow Blister Effect. The outlines of the shadow zones can be found by tracing the rays of light emitted by the outermost regions of the extended light source. The umbra region does not receive any direct light from any part of the light source, and is the darkest. A viewer located in the umbra region cannot directly see any part of the light source. By contrast, the penumbra is illuminated by some parts of the light source, giving it an intermediate level of light intensity. A viewer located in the penumbra region will see the light source, but it is partially blocked by the object casting the shadow. If there are multiple light sources, there will be multiple shadows, with overlapping parts darker, and various combinations of brightnesses or even colors. The more diffuse the lighting becomes, the softer and more indistinct the shadow outlines become, until they disappear. The lighting of an overcast sky produces few visible shadows. The absence of diffusing atmospheric effects in the vacuum of outer space produces shadows that are stark and sharply delineated by high-contrast boundaries between high and dark. For a person or object touching the surface where the shadow is projected (e.g. a person standing on the ground, or a pole in the ground) the multiple shadows converge at the point of contact. A shadow shows, apart from distortion, the same image as the silhouette when looking at the object from the sun-side, hence the mirror image of the silhouette seen from the other side. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Shadow」の詳細全文を読む スポンサード リンク
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