Shadow 217296 226079436 2008-07-16T19:04:21Z Cyclopia 88749 {{otheruses}} [[Image:People_Shadow.JPG|thumb|Shadows on the beach]] A '''shadow''' is a region of [[darkness]] where [[light]] is blocked by an object. It occupies all of the [[space]] behind an [[Opacity (optics)|opaque]] object with light in front of it. The [[cross section (geometry)|cross section]] of a shadow is a two-[[dimension]]al [[silhouette]], or reverse [[Projection (linear algebra)|projection]] of the object blocking the light. An astronomical object casts human visible shadows when its [[apparent magnitude]] is equal or lower than -4[http://www.gsfc.nasa.gov/scienceques2005/20060406.htm]. Currently the only astronomical objects able to cast shadows on Earth are the [[Sun]], the [[Moon]] and, in the right conditions, the planet [[Venus]]. ==Features== [[Image:Staxringold Coolshot 1.jpg|thumb|left|Shadows from a [[chandelier]]<br>showing umbra and penumbra]] [[Image:Poged_sa_tornja_na_senku.JPG|thumb|Shadow of the [[Fernsehturm]] in Berlin]] The closer to 90 degrees the angle between an elongated object and the direction of the light it blocks, the smaller 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. ==Time effect== Shadow lengths change dramatically by time. Unlike popular belief, shadows do not grow over the day. Instead, they reduce in lengths throughout the day. This is because the [[tangent]] of angle of the sun differs throughout the day. For instance, at 12 noon, when the sun is directly overhead, the shadow of a [[perpendicular]] object, say a 6-feet man, is actually undefined (as tan 90° = [[infinity]]). However, 4 minutes before noon, at 11.56am, his shadow would be 0.1 foot (as tan 89° = 57.289) and 4 minutes after noon, at 12.04pm, his shadow would be the abstract value -0.1 foot (as tan 91° = -57.289). Similarly, at [[sunrise]], about 6am, his shadow would be undefined (as tan 0° = 0). But 4 minutes later, at 6.04am, his shadow would have dramatically risen to 352.941 feet (as tan 1° = 0.017). Likewise, at [[sunset]], about 6pm, his shadow would be undefined (as tan 180° = 0). But 4 minutes earlier, at 5.56pm, his shadow would be the abstract value -352.941 feet (as tan 179° = -0.017). Abstract values (negative and undefined) of a shadow after noon indicates either it has shifted direction (from west to east) or that it has moved beyond the human [[spectrum]]. == Non-point == 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 coloured. 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 shadow or brighten 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. [[Image:British_Library_Gate_Shadow.jpg|thumb|Shadow of inverted text]] == Shadow propagation speed == The further the distance from the object blocking the light to the surface of projection, the larger the silhouette (they are considered [[proportional]]). Also, if the object is moving, the shadow cast by the object will project an image with dimensions (length) expanding proportionally faster than the object's own length of movement. The increase of size and movement is also true if the distance between the object of interference and the light source are closer. This, however, does not mean the shadow may move faster than light, even when projected at vast distances, such as [[light year]]s. The loss of light, which projects the shadow, will move towards the surface of projection at [[light speed]]. The projected shadow may appear to have moved faster than the speed of light, but there is no actual physical manifestation moving upon the surface. The misconception is that the edge of a shadow "moves" along a wall, when in actuality the increase of a shadow's length is part of a new projection, which will propagate at the speed of light from the object of interference. Since there is no actual communication between points in a shadow (except for reflection or interference of light, at the speed of light), a shadow that projects over a surface of large distances (light years) cannot give information between those distances with the shadow's edge. [http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3] ==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 photograph]]s, 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. ==Psychology== [[Shadow (psychology)|The Shadow]] side in [[Jungian psychology]] ([[Carl Jung]]), is a part of the [[unconscious mind]] consisting of [[Psychological repression|repressed]] weaknesses, shortcomings, and instincts. ==Mythological connotations== {{Expand-section|date=January 2007}} An unattended shadow or [[shade (mythology)|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. It is also believed as an alternative construct that shadows are in fact a representation of God's presence around an object; like a [[Halo (religious iconography)|halo]]. Early eastern beliefs also play to this theory. For example, [[Vishnu]] (a prominent [[Hindu]] god) would appear to help followers by assisting with tasks by lending some of his extra arms to assist the burden of the person. ==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 [[tincture (heraldry)|tincture]]s 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. ==See also== {{commons|Shadow}} *[[Trigonometry]] *[[Shade]] *[[Shadow play]] *[[Shadow people]] *[[gnomon]] [[Category:Optical phenomena]] [[gn:Kuarahy'ã]] [[bg:Сянка]] [[ca:Ombra]] [[cs:Stín]] [[da:Skygge]] [[de:Schatten]] [[el:Σκιά]] [[es:Sombra]] [[eo:Ombro]] [[fr:Ombre]] [[ko:그림자]] [[hi:प्रतिबिम्ब]] [[id:Bayang-bayang]] [[he:צל]] [[ka:ჩრდილი]] [[lv:Ēna]] [[lt:Šešėlis]] [[ml:നിഴല്‍]] [[nl:Schaduw]] [[ja:影]] [[nn:Skugge]] [[pl:Cień]] [[pt:Sombra]] [[ru:Тени (фотография)]] [[scn:Ùmmira]] [[simple:Shadow]] [[sk:Tieň]] [[sr:Сенка]] [[fi:Varjo]] [[te:నీడ]] [[tr:Gölge]] [[uk:Тінь]] [[zh:影]]