A
shadow is an area where direct
lightLight or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
from a light source cannot reach due to obstruction by an object. It occupies all of the
spaceSpace is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...
behind an
opaqueOpacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
object with light in front of it. The
cross sectionIn geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
of a shadow is a two-
dimensionIn physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...
al
silhouetteA silhouette is the image of a person, an object or scene consisting of the outline and a basically featureless interior, with the silhouetted object usually being black. Although the art form has been popular since the mid-18th century, the term “silhouette” was seldom used until the early decades...
, or reverse
projectionIn linear algebra and functional analysis, a projection is a linear transformation P from a vector space to itself such that P2 = P. It leaves its image unchanged....
of the object blocking the light. The sun causes many objects to have shadows and at certain times of the day, when the sun is at certain heights, the length of shadows change.
An
astronomical objectAstronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
casts human-visible shadows when its
apparent magnitudeThe apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
is equal or lower than −4. Currently the only astronomical objects able to produce visible shadows on Earth are the
sunThe Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, the
moonThe Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and, in the right conditions, the planet of
VenusVenus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
.
Variation with time
Shadow length changes dramatically throughout the day. The length of a shadow cast on the ground is proportional to the cotangent of the sun's elevation angle—its angle θ relative to the horizon. Near sunrise and sunset, when θ = 0° and cot(θ) is infinite, shadows can be extremely long. If the sun passes directly overhead, then θ = 90°, cot(θ)=0, and shadows are cast directly underneath objects.
Non-point
For a non-
point sourceA point source is a localised, relatively small source of something.Point source may also refer to:*Point source , a localised source of pollution**Point source water pollution, water pollution with a localized source...
of light, the shadow is divided into the
umbra and penumbraThe umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.These names are most often used to refer to the shadows cast by celestial bodies....
. The wider the light source, the more blurred the shadow. If two penumbras overlap, the shadows appear to attract and merge. This is known as the
Shadow Blister EffectThe Shadow Blister Effect is a visual effect whereby two adjacent shadows appear to bulge towards one another. It is the result of overlapping penumbra of two adjacent shadows on a reflecting surface, which originate from the same non-point light source...
.
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.
Shadow propagation speed
The farther the distance from the object blocking the light to the surface of projection, the larger the silhouette (they are considered
proportionalIn mathematics, two variable quantities are proportional if one of them is always the product of the other and a constant quantity, called the coefficient of proportionality or proportionality constant. In other words, are proportional if the ratio \tfrac yx is constant. We also say that one...
). 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 rate 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 years. The loss of light, which projects the shadow, will move towards the surface of projection at light speed.
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.
In photography
In photography, which is essentially recording patterns of light, shade, and colour, "highlights" and "shadows" are the brightest and darkest parts of a scene or image. Photographic exposure must be adjusted (unless special effects are wanted) to allow the
filmPhotographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
or
sensorA sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
, which has limited
dynamic rangeDynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
, to record detail in the highlights without them being washed out, and in the shadows without their becoming undifferentiated black areas.
Fog shadows
Fog shadows look odd since humans are not used to seeing shadows in three dimensions. The thin fog is just dense enough to be illuminated by the light that passes through the gaps in a structure or in a tree. As a result, the path of an object shadow through the "fog" appears darkened. In a sense, these shadow lanes are similar to
crepuscular raysCrepuscular rays , in atmospheric optics, are rays of sunlight that appear to radiate from a single point in the sky. These rays, which stream through gaps in clouds or between other objects, are columns of sunlit air separated by darker cloud-shadowed regions...
, which are caused by cloud shadows, but here, they are caused by the shadows of solid objects.
Other notes
A shadow cast by the
EarthEarth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
on the
MoonThe Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
is a
lunar eclipseA lunar eclipse occurs when the Moon passes behind the Earth so that the Earth blocks the Sun's rays from striking the Moon. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can only occur the night of a...
. Conversely, a shadow cast by the Moon on the Earth is a
solar eclipseAs seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...
.
On
satellite imagerySatellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History :The first images from space were taken on sub-orbital flights. The U.S-launched V-2 flight on October 24, 1946 took one image every 1.5 seconds...
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 imageA mirror image is a reflected duplication of an object that appears identical but reversed. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures...
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 shadowA rain shadow is a dry area on the lee side of a mountainous area. The mountains block the passage of rain-producing weather systems, casting a "shadow" of dryness behind them. As shown by the diagram to the right, the warm moist air is "pulled" by the prevailing winds over a mountain...
is a dry area, which, with respect to the prevailing
windWind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
direction, is beyond a
mountain rangeA mountain range is a single, large mass consisting of a succession of mountains or narrowly spaced mountain ridges, with or without peaks, closely related in position, direction, formation, and age; a component part of a mountain system or of a mountain chain...
; the range is "blocking" water from crossing the area. An
acoustic shadowAn acoustic shadow is an area through which sound waves fail to propagate, due to topographical obstructions or disruption of the waves via phenomena such as wind currents. A gobo refers to a movable acoustic isolation panel and that makes an acoustic shadow. As one website refers to it, "an...
can be created by terrain as well that will leave spots that can't easily hear sounds from a distance.
Sciophobia, or sciaphobia, is the fear of shadows.
Mythological connotations
An unattended shadow or
shadeIn traditional belief and fiction, a ghost is the soul or spirit of a deceased person or animal that can appear, in visible form or other manifestation, to the living. Descriptions of the apparition of ghosts vary widely from an invisible presence to translucent or barely visible wispy shapes, to...
was thought by some cultures to be similar to that of a ghost.
It is also believed as an alternative construct that shadows are in fact a representation of God's presence around an object; like a
haloA halo is a ring of light that surrounds a person in art. They have been used in the iconography of many religions to indicate holy or sacred figures, and have at various periods also been used in images of rulers or heroes...
. Early eastern beliefs also play to this theory.
(shadows aren't colored)
Heraldry
In
heraldryHeraldry is the profession, study, or art of creating, granting, and blazoning arms and ruling on questions of rank or protocol, as exercised by an officer of arms. Heraldry comes from Anglo-Norman herald, from the Germanic compound harja-waldaz, "army commander"...
, 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
tinctureIn heraldry, tinctures are the colours used to emblazon a coat of arms. These can be divided into several categories including light tinctures called metals, dark tinctures called colours, nonstandard colours called stains, furs, and "proper". A charge tinctured proper is coloured as it would be...
s different from the field on which it is placed), it is called
umbra-ted. Supposedly only a limited number of specific charges can be so depicted.
Shadows can be colored by a colored ransperent source of the shadow.
In popular culture
Speculative fictionSpeculative fiction is an umbrella term encompassing the more fantastical fiction genres, specifically science fiction, fantasy, horror, supernatural fiction, superhero fiction, utopian and dystopian fiction, apocalyptic and post-apocalyptic fiction, and alternate history in literature as well as...
occasionally features entities somehow made of shadow. Such beings are almost invariably evil.
See also
- Black drop effect
The black drop effect is an optical phenomenon visible during a transit of Venus and, to a lesser extent, a transit of Mercury.Just after second contact, and again just before third contact during the transit, a small black "teardrop" appears to connect Venus' disk to the limb of the Sun, making it...
- Convolution applications, for more physical and mathematical discussion about shadows
- Gnomon
The gnomon is the part of a sundial that casts the shadow. Gnomon is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals."It has come to be used for a variety of purposes in mathematics and other fields....
- Shade
Shade is the blocking of sunlight by any object, and also the shadow created by that object. Shade also consists of the colors grey, black, white, etc...
- Shadow Cabinet
The Shadow Cabinet is a senior group of opposition spokespeople in the Westminster system of government who together under the leadership of the Leader of the Opposition form an alternative cabinet to the government's, whose members shadow or mark each individual member of the government...
- Shadow mapping
Shadow mapping or projective shadowing is a process by which shadows are added to 3D computer graphics. This concept was introduced by Lance Williams in 1978, in a paper entitled "Casting curved shadows on curved surfaces"...
, in computer 3D graphics
- Shadow people
Shadow people are supernatural shadow-like humanoid figures that, according to believers, are seen mostly in peripheral vision and move quickly...
- Shadow play
Shadow play or shadow puppetry Shadow puppets have a long history in China, India, Turkey and Java, and as a popular form of entertainment for both children and adults in many countries around the world. A shadow puppet is a cut-out figure held between a source of light and a translucent screen...
External links