Shadow

:This article is about the optical phenomenon; for other meanings, see Shadow (disambiguation).

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A shadow is a dark shape, e.g. on the ground or a wall, caused by an object (or person, etc.) blocking light. Like a silhouette, the shape of the shadow is a two-dimensional projection of the object, but:

Related Topics:
Light - Silhouette - Dimension - Projection

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• the smaller the angle between an elongated object and the direction of the light is, the shorter the shadow is;
• the smaller the angle between the direction of the light and the surface on which the shadow occurs is, the longer the shadow is;
• if the object is close to the light source, the shadow is large.

If the surface is curved there are further distortions.

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For non-point sources of light, the shadow is divided into the umbra and penumbra. The wider the light source, the more blurred the shadow.

Related Topics:
Umbra - Penumbra

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If there are multiple light sources there are multiple shadows, with overlapping parts darker. For a person or object touching the surface, like a person standing on the ground, or a pole in the ground, these converge at the point of touch.

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If white light is produced by separate colored light sources, the shadows are colored.

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A shadow cast by the Earth on the Moon is a lunar eclipse. Conversely, a shadow cast by the Moon on the Earth is a solar eclipse.

Related Topics:
Earth - Moon - Eclipse

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It is also true that shadows can travel faster than the speed of light. A shadow 'moves' faster than the object that casts it. If the object is moving at near lightspeed, its shadow can move faster than the speed of light. However, shadows cannot carry information so this phenomenon does not disobey general relativity.

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On satellite imagery and aerial photographs, taken vertically, tall buildings can be recognized as such by their long shadows (if the photographs are not taken in the tropics around noon) , while these also show more of the shape of these buildings.

Related Topics:
Satellite imagery - Aerial photograph

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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 (see picture).

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The term shadow is also used with regard to other things than light, for example rain: a rain shadow is a dry area, which, with respect to the prevailing wind direction, is beyond a mountain range; it is dry because air masses lose part of their water when they move over these mountains.

Related Topics:
Rain - Rain shadow - Wind - Mountain range - Air - Water

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