shadow
Shadow is
- the unlit area of an object ("shadow side": own shadow , body shadow )
- by means of a light source generated projection of an object on other objects ( "cast a shadow": Shadow, silhouette ).
- more generally the entire unlit space behind an illuminated body ("in the shadow") - this space extends within the boundaries of the shaded side up to the shadow
Depending on the number and extent of the light sources and various optical effects, a distinction is made between several types of shadow (umbra, penumbra, mixed shadow) . The edge of the unlit shadow is called the shadow border or light-dark border , in astronomy and meteorology also terminator or day-night border .
Shadows from a single light source
Own shadow
A body's own shadow (body shadow) is the shadow that the body creates on itself by itself. However, only shadow areas are meant that lie on the areas causing the shadow. In other words, the own shadow is the number of non-illuminated sides, i.e. the sides facing away from the light source.
A shadow is the lack of radiation behind an object, which the object receives on its side facing the radiation source. A shadow is always cast by an object in the opposite direction to the radiation it has received, even if this shadow does not hit another object.
Drop shadow
The drop shadow is created on a light background when the object in front of it is illuminated by an almost point-like light source (sun, spotlight, photo flash ). This shadow is remarkably sharp and is therefore perceived particularly intensely by the viewer. It depicts the object at least partially and, like the body shadow, shows the direction from which the light is coming.
The shadow projection is also called silhouette , it shows the outline, i.e. the silhouette of the object. A silhouette is also called a drawing that is created by tracing the shadow projection. In shadow theater , the enlargement of the shadow can be used to change the apparent proportions of two objects to one another by positioning them differently close to the light source.
This shadow effect has a negative aspect in connection with very strong exposure events , such as those caused by the atomic bomb flash. The enormous radiation density causes thermal material changes in the unshaded area, whereby the little irradiated shadow area leaves a contrast as a permanent image.
Geometry of the shadow
A real hard shadow only exists under idealized conditions , i.e. a point light source, apart from these only black (reflectionless) bodies , and in a vacuum .
Then the shadow space is the volume between the body's own shadow and its silhouette, the points of the shadow border are connected in a straight line with their respective projections on the outline of the drop shadow. These lines diverge conically from the point source ( light cone ) , so that the shadow is larger than the shadow-casting body, the larger the further it is from the projection surface, the smaller the further it is from the light source. The lines are only parallel when the light source is infinitely distant (for questions on earth, the sun is to be assumed as "infinitely distant").
In descriptive geometry , shadows from spatial objects (house, tower, bridge, etc.) are used to make images more attractive or to identify lighting and exposure problems in the planning phase. Shadow constructions for shadows from parallel (sun) or central (lamp) light on a horizontal plane are relatively simple for both parallel projections and central projections. Complicated cases are left to the computer today, since modern image synthesis (rendering) is based on ray tracing anyway .
Shadows with extended, multiple or diffuse light sources
Umbra
The umbra is the space behind an illuminated object that is not illuminated by any light source.
Since real light sources are not punctiform, but have a certain spatial extent, the outlines of a shadow are not sharply delimited. The reason for this is that parts of the light source are covered at the edge of the shadow, but other areas of the light source are still visible (penumbra) . If the light source is small enough or sufficiently far away, however, there is an area inside the shadow in which the light source is completely covered. This area is the umbra. If the light source is larger than the body that casts the shadow, its projection lines converge conically (in the case of a round body they form a cone). Therefore, the umbra only reaches a certain distance behind the shadow-casting body.
During solar eclipses you experience a total solar eclipse when you are in the umbra of the moon. Since the moon appears about the same size as the sun when viewed from the earth , the umbra reaches the earth almost point-like. In penumbra you only experience a partial or a ring-shaped solar eclipse. In the latter case, the umbra does not reach the earth.
A special effect is the antumbra , sometimes referred to in German as "Gegenenschatten" or - correctly translated - "Voratten" (from Latin ante "before"). It occurs when a body obscures an extended source of light that is larger than the body itself. At a sufficient distance, the outline of the body appears completely in front of the outline of the light source without completely obscuring the light source. Thus the Antumbra is the centrally located continuation of the umbra. For example, the viewer of a ring-shaped solar eclipse is in the antrum of the moon. For the same reason, the inner planets (or other celestial bodies, including exoplanets ) only move as a black disk over the surface of the sun, and one speaks of transit , not "darkness" (the solar eclipse is the transit of the moon before the sun, the lunar eclipse is the transit of the earth, seen from the moon).
Penumbra
The penumbra is the space behind an illuminated object that is not illuminated by all light sources in the surroundings (mixed shadow) . A single point light source can therefore adhoc not cause a penumbra behind a shadow-casting object. Only the existence of at least one second point source can an umbra and around it generate a maximum of as many penumbra areas as there are light sources.
However, the most common cause of penumbra is extensive light sources. If, for example, a matted luminous body is used to illuminate a room, an almost black shadow area can be seen in the shadow of a body on the wall and a penumbra area around it. If you were to look from the area of the umbra towards the light source, it would be completely covered by the object. In contrast, the light source is only partially covered from the penumbra. The darker area of the umbra is more extensive and more sharply defined the closer the object is to the wall. As the distance between the object and the wall increases, the umbra disappears and only penumbra remains.
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In contrast to the above, there is a penumbra due to various optical effects even with only one light source. The main reason is refraction . A hole creates a penumbra in the shape of the light source if the hole forms a smaller solid angle than the light source (refraction at the gap). This can be observed in nature with circular images of the sun under trees - even if the gaps in the canopy have an angular shape, the light spots are round. During a partial solar eclipse, these "sun circles" or "sun talers" change their shape from rounded spots to curved sickles. The effect can be found in perfected form in the camera obscura .
But there is also refraction in the Earth's atmosphere , for example , so that it only casts a pronounced penumbra from the sun. Therefore, a lunar eclipse is less complete than a solar eclipse, where the hard shadow of the atmospheric moon hits the earth. In the case of the lunar eclipse, the effect of the penumbra is practically invisible to the naked eye (and since the inner penumbra is created by the same refraction that creates the evening and morning red, it is also illuminated in the umbra and reddish: "copper moon"). Conversely, however, the new moon (shadowed side faces the earth) is not completely dark because the reflection of the earth as a second light source is quite powerful.
In horticulture, penumbra means when a plant is not directly exposed to the sun. During the main shoot season, April and May, there is light shade, possibly tree shade for half a day in summer, and light shade again in late autumn. In general, refractions and reflections also play a role, so that the shadow on earth is never completely "lightless". The more diffuse the lighting through the surroundings, the "softer" the shadow. That is why there is hardly a shadow in diffuse, cloudy weather. Conversely, however, in a diffuse atmosphere (moisture, dust), the volume of the shadow itself is recognizable because light components are also scattered out here ( shadow rays )
One of the earliest artistic depictions of core and penumbra can be found in Robert Campin (painting of the “Gnadenstuhl”, Städel Frankfurt, approx. 1410–30).
Shadows from different light sources
Sun, moon and lightning bolts at night can create visible shadows. Artificial light sources such as lamps, lightbulbs, car headlights or candles also create clearly visible shadows. However, shadows can only be seen if the light source is so strong that a light reflection can still be perceived outside the shadow. Stars are so faint that they produce no visible shadow. Even Sirius , the brightest star in the sky, isn't bright enough to cast a shadow. Occasionally there is speculation about whether Venus is bright enough to create a visible shadow. This might actually be possible - under special circumstances. Anyone who walks through a snowy landscape with perfectly adapted eyes on a cloudless, clear new moon night at the time of the greatest Venus brightness, far away from all artificial light sources , has the chance to see shadows created by Venus.
Colored shadows
Colored shadows arise when a scene is illuminated by at least two differently colored light sources, in the area of the penumbra that the individual illuminated objects cast on one another and on the background or background of the scene. They can also be created with a single, monochrome light source and a colored, transparent object, for example a colored glass.
Creation of colored shadows
Through the controlled use of differently colored light sources and objects positioned accordingly in the scene, colored shadows can be experimentally created on a projection screen. The perceived mixed colors follow the laws of additive color mixing .
Colored shadows in natural lighting
In natural lighting, interactions with the inherent color generated by subtractive color mixing (e.g. the paint ) of the background occur in the penumbra area :
When illuminated with daylight , the shadow areas are not completely dark, but are brightened by scattered light from the sky blue . This is spectrally blue-shifted compared to primary sunlight ( Rayleigh scattering ), which is why shadow areas of a surface of the same color contain a higher proportion of blue compared to the areas that are in direct sunlight. This becomes particularly clear in the case of diffusely reflective surfaces, such as a (colored) plastered house wall or a (pure white) fresh snow surface.
In his theory of colors, Goethe dealt intensively with these natural colored shadows and looked for reasons why they (also according to observations by Horace-Bénédict de Saussure ) do not always have a bluish cast.
Colored shadows as a pictorial means of creation
The precise observation of the color nuances in colored shadows and other effects of the atmospheric optics was and is the starting point for the development of important artistic stylistic devices, especially in impressionism . The depiction of the interaction between the depicted object and the spatial surrounding atmosphere creates an image space that includes the observer and gives the images immediate liveliness and freshness.
Tools
A horizonoscope is a quick and easy tool to determine on-site at which times of day and seasons obstacles cast a shadow on a point.
See also
- Shadow effect , shadow (computer graphics) , the representation of shadows
- Sun protection (architecture) , shading in architecture
- Splash shadow
- Shading , X-ray technology
- Shading analysis , solar technology
Web links
- Experimental arrangement for creating colored shadows ( LEIFI )
- Section from Goethe's theory of colors on the subject of "colored shadows"
Individual evidence
- ^ Stefan Krause: Solar eclipse in Australia. BoD - Books on Demand, 2012, ISBN 978-3-848-22321-3 , p. 163 ( limited preview in Google book search).