A shadow is a dark area where light from a light source is blocked by an 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 a reverse projection of the object blocking the light.
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 is more than one light source, there will be several shadows, with the overlapping parts darker, and various combinations of brightnesses or even colors. The more diffuse the lighting is, 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 light 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 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.
The names umbra, penumbra and antumbra are often used for the shadows cast by astronomical objects, though they are sometimes used to describe levels of darkness, such as in sunspots. An astronomical object casts human-visible shadows when its apparent magnitude is equal or lower than -4. [2] The only astronomical objects able to project visible shadows onto Earth are the Sun, the Moon, and in the right conditions, Venus or Jupiter. [3] Night is caused by the hemisphere of a planet facing its orbital star blocking its sunlight.
A shadow cast by the Earth onto the Moon is a lunar eclipse. Conversely, a shadow cast by the Moon onto the Earth is a solar eclipse. [4]
The sun casts shadows that change dramatically through 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) = ∞, shadows can be extremely long. If the sun passes directly overhead (only possible in locations between the Tropics of Cancer and Capricorn), then θ = 90°, cot(θ) = 0, and shadows are cast directly underneath objects.
Such variations have long aided travellers during their travels, especially in barren regions such as the Arabian Desert. [5]
The farther 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 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.
Although the edge of a shadow appears to "move" along a wall, in actuality the increase of a shadow's length is part of a new projection that propagates 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 convey information between those distances with the shadow's edge. [6]
Visual artists are usually very aware of colored light emitted or reflected from several sources, which can generate complex multicolored shadows. Chiaroscuro, sfumato, and silhouette are examples of artistic techniques which make deliberate use of shadow effects. [7]
During the daytime, a shadow cast by an opaque object illuminated by sunlight has a bluish tinge. This happens because of Rayleigh scattering, the same property that causes the sky to appear blue. The opaque object is able to block the light of the sun, but not the ambient light of the sky which is blue as the atmosphere molecules scatter blue light more effectively. As a result, the shadow appears bluish. [8]
A shadow occupies a three-dimensional volume of space, but this is usually not visible until it projects onto a reflective surface. A light fog, mist, or dust cloud can reveal the 3D presence of volumetric patterns in light and shadow.
Fog shadows may look odd to viewers who are not used to seeing shadows in three dimensions. A 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's shadow through the fog becomes visible as a darkened volume. In a sense, these shadow lanes are the inverse of crepuscular rays caused by beams of light, they're caused by the shadows of solid objects.
Theatrical fog and strong beams of light are sometimes used by lighting designers and visual artists who seek to highlight three-dimensional aspects of their work.
Oftentimes shadows of chain-linked fences and other such objects become inverted (light and dark areas are swapped) as they get farther from the object. A chain-link fence shadow will start with light diamonds and shadow outlines when it is touching the fence, but it will gradually blur. Eventually, if the fence is tall enough, the light pattern will go to shadow diamonds and light outlines.
In photography, which is essentially recording patterns of light, shade, and color, "highlights" and "shadows" are the brightest and darkest parts, respectively, of a scene or image. Photographic exposure must be adjusted (unless special effects are wanted) to allow the film or sensor, which has limited dynamic range, to record detail in the highlights without them being washed out, and in the shadows without their becoming undifferentiated black areas.
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.
Shadow as a term is often used for any occlusion or blockage, 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 elevated terrain impedes rainclouds from entering the dry zone. An acoustic shadow occurs when a direct sound has been blocked or diverted around a given area.
Shadows often appear in mythical or cultural contexts. Sometimes in a malevolent light, other times not. An unattended shade was thought by some cultures to be similar to that of a ghost. The name for the fear of shadows is "sciophobia" or "sciaphobia".
Chhaya is the Hindu goddess of shadows.
In heraldry, when a charge is supposedly shown "in the 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 technically described as "umbrated". Supposedly, only a limited number of specific charges can be so depicted. [ citation needed ]
Shadows are often linked with darkness and evil; in common folklore, like shadows who come to life, are often evil beings trying to control the people they reflect. The film Upside-Down Magic features an antagonistic shadow spirit who possesses people.
Ancient Egyptians surmised that a shadow, which they called šwt (shut), contains something of the person it represents because it is always present. Through this association, statues of people and deities were sometimes referred to as shadows.
In a commentary to The Egyptian Book of the Dead (BD), Egyptologist Ogden Goelet, Jr. discusses the forms of the shadow: "In many BD papyri and tombs the deceased is depicted emerging from the tomb by day in shadow form, a thin, black, featureless silhouette of a person. The person in this form is, as we would put it, a mere shadow of his former existence, yet nonetheless still existing. Another form the shadow assumes in the BD, especially in connection with gods, is an ostrich-feather sun-shade, an object which would create a shadow." [9]
Scientists from the National University of Singapore presented a shadow-effect energy generator (SEG), which consists of cells of gold deposited on a silicon wafer attached on a plastic film. The generator has a power density of 0.14 μW cm−2 under indoor conditions (0.001 sun). [10]
An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three celestial objects is known as a syzygy. An eclipse is the result of either an occultation or a transit. A "deep eclipse" is when a small astronomical object is behind a bigger one.
A lunar eclipse is an astronomical event that occurs when the Moon moves into the Earth's shadow, causing the Moon to be darkened. Such an alignment occurs during an eclipse season, approximately every six months, during the full moon phase, when the Moon's orbital plane is closest to the plane of the Earth's orbit.
The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a collimated beam of light, only the umbra is cast.
Shading refers to the depiction of depth perception in 3D models or illustrations by varying the level of darkness. Shading tries to approximate local behavior of light on the object's surface and is not to be confused with techniques of adding shadows, such as shadow mapping or shadow volumes, which fall under global behavior of light.
Shadow volume is a technique used in 3D computer graphics to add shadows to a rendered scene. They were first proposed by Frank Crow in 1977 as the geometry describing the 3D shape of the region occluded from a light source. A shadow volume divides the virtual world in two: areas that are in shadow and areas that are not.
Ray casting is the methodological basis for 3D CAD/CAM solid modeling and image rendering. It is essentially the same as ray tracing for computer graphics where virtual light rays are "cast" or "traced" on their path from the focal point of a camera through each pixel in the camera sensor to determine what is visible along the ray in the 3D scene. The term "Ray Casting" was introduced by Scott Roth while at the General Motors Research Labs from 1978–1980. His paper, "Ray Casting for Modeling Solids", describes modeled solid objects by combining primitive solids, such as blocks and cylinders, using the set operators union (+), intersection (&), and difference (-). The general idea of using these binary operators for solid modeling is largely due to Voelcker and Requicha's geometric modelling group at the University of Rochester. See solid modeling for a broad overview of solid modeling methods. This figure on the right shows a U-Joint modeled from cylinders and blocks in a binary tree using Roth's ray casting system in 1979.
Whiteout, white-out, or milky weather is a weather condition in which the contours and landmarks in a snow-covered zone become almost indistinguishable. It could be also applied when visibility and contours are greatly reduced by sand. The horizon disappears from view while the sky and landscape appear featureless, leaving no points of visual reference by which to navigate; there is absence of shadows because the light arrives in equal measure from all possible directions. Whiteout has been defined as: "A condition of diffuse light when no shadows are cast, due to a continuous white cloud layer appearing to merge with the white snow surface. No surface irregularities of the snow are visible, but a dark object may be clearly seen. There is no visible horizon."
A gobo is an object placed inside or in front of a light source to control the shape of the emitted light and its shadow.
Shadow mapping or shadowing projection 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." Since then, it has been used both in pre-rendered and realtime scenes in many console and PC games.
The magnitude of eclipse is the fraction of the angular diameter of a celestial body being eclipsed. This applies to all celestial eclipses. The magnitude of a partial or annular solar eclipse is always between 0.0 and 1.0, while the magnitude of a total solar eclipse is always greater than or equal to 1.0, and has a theoretically maximum value of around 1.12.
The umbra is the innermost and darkest part of a shadow, where the light source is completely blocked by the occluding body.
A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season in its new moon phase, when the Moon's orbital plane is closest to the plane of the Earth's orbit. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured. Unlike a lunar eclipse, which may be viewed from anywhere on the night side of Earth, a solar eclipse can only be viewed from a relatively small area of the world. As such, although total solar eclipses occur somewhere on Earth every 18 months on average, they recur at any given place only once every 360 to 410 years.
The penumbra is the part of a shadow where the light source is only partially blocked.
A partial lunar eclipse took place on Saturday, July 6, 1963 with an umbral eclipse magnitude of 0.70602. The Moon was strikingly shadowed in this deep partial eclipse which lasted 3 hours exactly, with 71% of the Moon in darkness at maximum. A partial lunar eclipse occurs when the Earth moves between the Sun and Moon but the three celestial bodies do not form a straight line in space. When that happens, a small part of the Moon's surface is covered by the darkest, central part of the Earth's shadow, called the umbra. The rest of the Moon is covered by the outer part of the Earth's shadow called the penumbra. It was the second of three lunar eclipses in 1963, the first was a penumbral lunar eclipse on January 9, 1963 and the third and last was on December 30, 1963.
A partial lunar eclipse took place on Tuesday, November 29, 1955 with an umbral eclipse magnitude of 0.11899. A partial lunar eclipse happens when the Earth moves between the Sun and the Full Moon, but they are not precisely aligned. Only part of the Moon's visible surface moves into the dark part of the Earth's shadow. A partial lunar eclipse occurs when the Earth moves between the Sun and Moon but the three celestial bodies do not form a straight line in space. When that happens, a small part of the Moon's surface is covered by the darkest, central part of the Earth's shadow, called the umbra. The rest of the Moon is covered by the outer part of the Earth's shadow called the penumbra. It was the second of two lunar eclipses in 1955, first being the penumbral lunar eclipse on June 5. It also occurred near perigee, making such event a supermoon.
A penumbral lunar eclipse took place at the Moon's ascending of the orbit on Sunday, June 5, 1955, with a penumbral eclipse magnitude of 0.62181 (62.181%). A penumbral lunar eclipse takes place when the Moon moves through the faint, outer part of Earth's shadow, the penumbra. This type of eclipse is not as dramatic as other types of lunar eclipses and is often mistaken for a regular Full Moon. The Moon shines because its surface reflects the Sun's rays. A lunar eclipse happens when the Earth comes between the Sun and the Moon and blocks some or all of the Sun's light from reaching the Moon. A penumbral lunar eclipse occurs when the Sun, Earth, and the Moon are imperfectly aligned. When this happens, the Earth blocks some of the Sun's light from directly reaching the Moon's surface and covers all or part of the Moon with the outer part of its shadow, also known as the penumbra. Since the penumbra is much fainter than the dark core of the Earth's shadow, the umbra, a penumbral eclipse of the Moon is often difficult to tell apart from a normal Full Moon. Occurring only 0.5 days after apogee, the moon's apparent diameter was 6.5% smaller than average.
The earliest scientifically useful photograph of a total solar eclipse was made by Julius Berkowski at the Royal Observatory in Königsberg, Prussia, on July 28, 1851. This was the first occasion that an accurate photographic image of a solar eclipse was recorded.
Earth's shadow is the shadow that Earth itself casts through its atmosphere and into outer space, toward the antisolar point. During the twilight period, the shadow's visible fringe – sometimes called the dark segment or twilight wedge – appears as a dark and diffuse band just above the horizon, most distinct when the sky is clear.
The shadow blister effect is a visual phenomenon in which a shadow bulges as it approaches another.
Solar eclipses on the Moon are caused when the planet Earth passes in front of the Sun and blocks its light. Viewers on Earth experience a lunar eclipse during a solar eclipse on the Moon.
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