![Cumulus cloud shadows 1](https://upload.wikimedia.org/wikipedia/commons/b/b4/Cumulus_cloud_shadows_1.jpg)
A shadow is a dark area where
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
from a light source is blocked by an
opaque
Opacity or opaque may refer to:
* Impediments to (especially, visible) light:
** Opacities, absorption coefficients
** Opacity (optics), property or degree of blocking the transmission of light
* Metaphors derived from literal optics:
** In lingu ...
object. It occupies all of the three-dimensional volume behind an object with light in front of it. The
cross section
Cross section may refer to:
* Cross section (geometry)
** Cross-sectional views in architecture & engineering 3D
*Cross section (geology)
* Cross section (electronics)
* Radar cross section, measure of detectability
* Cross section (physics)
**Abs ...
of a shadow is a two-
dimension
In physics and mathematics, the dimension of a mathematical 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 (1D) because only one coor ...
al
silhouette
A silhouette ( , ) is the image of a person, animal, object or scene represented as a solid shape of a single colour, usually black, with its edges matching the outline of the subject. The interior of a silhouette is featureless, and the silhou ...
, or a reverse
projection
Projection, projections or projective may refer to:
Physics
* Projection (physics), the action/process of light, heat, or sound reflecting from a surface to another in a different direction
* The display of images by a projector
Optics, graphic ...
of the object blocking the light.
Point and non-point light sources
![Diagram of umbra, penumbra & antumbra](https://upload.wikimedia.org/wikipedia/commons/2/2e/Diagram_of_umbra%2C_penumbra_%26_antumbra.png)
A
point source
A point source is a single identifiable ''localised'' source of something. A point source has negligible extent, distinguishing it from other source geometries. Sources are called point sources because in mathematical modeling, these sources ca ...
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
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 during an eclipse season, approximately every six month ...
. 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 shadow blister effect is a visual phenomenon in which a shadow bulges (or blisters) as it approaches another.
The effect takes place when two objects are at varying distances between a non- point light source and a background upon which th ...
.
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
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
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
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
of
outer space
Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, pred ...
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.
Astronomy
![Three moons and their shadows parade across Jupiter](https://upload.wikimedia.org/wikipedia/commons/8/85/Three_moons_and_their_shadows_parade_across_Jupiter.jpg)
The names
umbra, penumbra and antumbra
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 (such as a point source) of light, only the umbra is cast.
Th ...
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
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...
is equal or lower than -4. The only astronomical objects able to project visible shadows onto Earth are the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, and in the right conditions,
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
or
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
.
Night
Night (also described as night time, unconventionally spelled as "nite") is the period of ambient darkness from sunset to sunrise during each 24-hour day, when the Sun is below the horizon. The exact time when night begins and ends depends ...
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.
Daytime variation
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
In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the ''radial distance'' of that point from a fixed origin, its ''polar angle'' measu ...
—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.
Propagation speed
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
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...
.
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.
Color
Visual artists are usually very aware of colored light emitted or reflected from several sources, which can generate complex multicolored shadows.
Chiaroscuro
Chiaroscuro ( , ; ), in art, is the use of strong contrasts between light and dark, usually bold contrasts affecting a whole composition. It is also a technical term used by artists and art historians for the use of contrasts of light to achi ...
,
sfumato, and
silhouette
A silhouette ( , ) is the image of a person, animal, object or scene represented as a solid shape of a single colour, usually black, with its edges matching the outline of the subject. The interior of a silhouette is featureless, and the silhou ...
are examples of artistic techniques which make deliberate use of shadow effects.
During the daytime, a shadow cast by an opaque object illuminated by sunlight has a bluish tinge. This happens because of
Rayleigh scattering
Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of th ...
, 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.
Dimension
![Fog shadow of GGB](https://upload.wikimedia.org/wikipedia/commons/9/96/Fog_shadow_of_GGB.jpg)
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
Crepuscular rays are sunbeams that originate when the Sun is just below the horizon, during the twilight period. Crepuscular rays are noticeable when the contrast between light and dark is most obvious. Crepuscular comes from the Latin word ...
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.
Inversion
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.
Photography
![Moonlight shadow](https://upload.wikimedia.org/wikipedia/commons/2/21/Moonlight_shadow.jpg)
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
In photography, exposure is the amount of light per unit area (the image plane's illuminance times the exposure time) reaching a frame of photographic film or the surface of an electronic image sensor, as determined by shutter speed, lens ...
must be adjusted (unless special effects are wanted) to allow the
film or
sensor, which has limited
dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of Signal (electrical engineering), signals, like sound and light. It is measured ...
, 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 photograph
Aerial photography (or airborne imagery) is the taking of photographs from an aircraft or other airborne platforms. When taking motion pictures, it is also known as aerial videography.
Platforms for aerial photography include fixed-wing ai ...
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.
Analogous concepts
''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
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ho ...
direction, is beyond a
mountain range
A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have arise ...
; the elevated terrain impedes rainclouds from entering the dry zone. An
acoustic shadow An acoustic shadow or sound 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, buildings, or sound barriers.
Short-distance acoustic shad ...
occurs when a direct sound has been blocked or diverted around a given area.
Cultural aspects
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
A tincture is typically an extract of plant or animal material dissolved in ethanol (ethyl alcohol). Solvent concentrations of 25–60% are common, but may run as high as 90%.Groot Handboek Geneeskrachtige Planten by Geert Verhelst In chemis ...
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.
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."
Energy generating
Scientists from the
National University of Singapore presented a shadow-effect energy generator (SEG), which consists of cells of
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...
deposited on a
silicon wafer
In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serv ...
attached on a plastic film. The generator has a
power density
Power density is the amount of power (time rate of energy transfer) per unit volume.
In energy transformers including batteries, fuel cells, motors, power supply units etc., power density refers to a volume, where it is often called volum ...
of 0.14 μW cm
−2 under indoor conditions (0.001 sun).
Gallery
File: Apollo 9 astronaut Dave Scott.jpg, Non-diffuse lighting in outer space causes deep shadows
File: British Library Gate Shadow.jpg, Reversed text in shadow
File:Fog shadow tv tower.jpg, Sutro Tower
Sutro Tower is a unique three-legged tall TV and radio lattice tower located in San Francisco, California. Rising from a hill between Twin Peaks and Mount Sutro near Clarendon Heights, it is a prominent feature of the city skyline and a landma ...
casts a 3D fog shadow
File: Jasminum sambac1SHSU.jpg, This photo of jasmine flowers has only soft shadows cast by diffused light
File:CloudsShadow.jpg, Clouds, and shadows over the Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...
File: Vapour shadow.jpg, Shadow cast by vapor trail
Contrails (; short for "condensation trails") or vapor trails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several miles above the Earth's surface. Contrails ar ...
of passing aircraft
File: Shadow of parent and child.jpg, Shadow of a parent and child
File: Footpath in Richmond, Surrey - UK.jpg, Tree shadow
File: Erőd - Trsat, Fiume (7).jpg, Shadow on the Castle
File:Shadow, Bukit, Bali, Indonesia.jpg, Moonlight shadow
File: Long shadow of a dead tree with its branches on the dry fields of Laos - portrait.jpg, Long shadow of a dead tree with its branches on dry fields, late afternoon
File: Skugga - Ystad-2020.jpg, When the sun is low, shadows become long, and details get the wrong proportions.
File:Klädhängare - KappAhl - 2022.jpg, Shadows from a window blind
A window blind is a type of window covering. There are many different kinds of window blinds which use a variety of control systems. A typical window blind is made up of several long horizontal or vertical slats of various types of hard mate ...
.
See also
*
Raking light
Raking light, the illumination of objects from a light source at an oblique angle or almost parallel to the surface, provides information on the surface topography and relief of the artefact thus lit. It is widely used in the examination of wor ...
*
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.
Description
Just after astronomical transit#Contacts, second contact, and again just before astronomical transit#Con ...
*
Convolution applications, for more physical and mathematical discussion about shadows
*
Earth's shadow
*
Gnomon
*
Shade
*
Shadow Cabinet
*
Shadow mapping
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 b ...
, in computer 3D graphics
*
Shadow people
A shadow person (also known as a shadow figure or black mass) is the perception of a patch of shadow as a living, humanoid figure, and interpreted as the presence of a spirit or other entity by believers in the paranormal or supernatural.
His ...
*
Shadow play
Shadow play, also known as shadow puppetry, is an ancient form of storytelling and entertainment which uses flat articulated cut-out figures (shadow puppets) which are held between a source of light and a translucent screen or scrim. The cut-o ...
*
Shadowgraphy or
ombromanie
Shadowgraphy or ombromanie is the art of performing a story or show using images made by hand shadows. It can be called "cinema in silhouette". Performers are titled as a shadowgraphist or shadowgrapher.
The art has declined since the late 19th ...
, the art of hand shadows
*
Sciography
Sciography, also spelled sciagraphy or skiagraphy ( el, σκιά "shadow" and γράφειν graphein, "write"), is a branch of science of the perspective dealing with the projection of shadows, or delineation of an object in perspective with it ...
, the art of architectural shadows
*
Shadows in camouflage
References
External links
Art project about shadows and their philosophical connotationsHow sun casts shadows over day hours
{{Authority control
Optical phenomena