A shadow is a region of darkness where light is blocked. A shadow occupies all the space behind an opaque 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.
Features
showing umbra and penumbra
The closer to 90 degrees the angle between an elongated object and the direction of the light it blocks, the bigger its shadow. The smaller the angle between the direction of the light and the surface on which the shadow occurs, 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.
For a non-point source of light, the shadow is divided into the umbra and penumbra. The wider the light source, the more blurred the shadow.
If there are multiple light sources there are multiple shadows, with overlapping parts darker, or a combination of colors. 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.
Colored shadows
If white light is produced by separate colored light sources, the shadows are colored.
Illuminate a room with a red light, and the shadows are exclusively gray, or dark. Illuminate the shadows with a white light, and the shadows are green. Where both lights are blocked, or in other words where the shadows intersect, the shadows are gray. Away from the intersection, where the red light is blocked the shadows are green, and where the white light is blocked the shadows are red. In other words, light colors shadows or brightens them, according to the complementary color of the light blocked to cast the shadow. In the case of white and red lights, the complement of white is red; with white and green lights, the complement of white is green.
In the absence of multiple light sources, colored lights illuminate spaces where other lights are not blocked. In the above example, the red shadow cast by blocking white light is not a shadow with the white light off, but it is illuminated in red.
In the absence of white light, colored lights blocked by an opaque surface cast shadows in the colors complementary to the lights blocked. For green light, red shadows, and vice-versa; blue, orange; yellow, purple; intermediate light, intermediate shadows.
Faster than light
For objects moving at every-day speeds (much slower than the speed of light), the shadow cast by an object will move faster than the object which casts it. A cross-section of a shadow (a silhouette) is displaced by the motion of an object in front of a point source of light. The further the distance from the object blocking the light, the larger the silhouette and the greater the displacement by motion.
That's a boy?
However, this simple relationship between speeds and distances becomes more complicated over vast distances for very fast moving objects due to the finite speed of light; the motion of an object may cut off the emission of light from a source to a surface, but the light that had already passed by the object will take some time before reaching the surface, and so there is some delay before the shadow on the surface reflects the updated position of the object. Thus, while it is certainly possible to create shadows that move faster than light[1], this effect cannot be used to transmit information at superluminal speeds, because the motion of the shadow is being caused by the motion of the object in the past, not the present.
Other notes
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.
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.
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).
Shadow as a term is often used for any occlusion, not just those with respect to light. For example, a rain shadow is a dry area, which, with respect to the prevailing wind direction, is beyond a mountain range; the range is "blocking" water from crossing the area. An acoustic shadow can be created by terrain as well that will leave spots that can't easily hear sounds from a distance.
Mythological connotations
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An unattended shadow or shade was thought by some cultures to be similar to that of a ghost, a flicker of a life unable to end for some reason.
In many works of modern fantasy, shadows are often intertwined with dark arts and black magic.
Heraldry
In heraldry, when a charge is supposedly shown in shadow (the appearance is of the charge merely being outlined in a neutral tint rather than being of one or more tinctures different from the field on which it is placed), it is called umbrated. Supposedly only a limited number of specific charges can be so depicted.
Shadows are dangerous because if you walk around with a gun, and you see your shadow, and think its a stalker and you shoot it, the bullets will bounce back at you and kill you. so shadows can kill.
See also
