影

  关于与「影」標題相近或相同的条目页，請見「影 (消歧義)」。

從 艾菲爾鐵塔平臺上，往下看到的遊客影子。

雪地表面的不平坦導致公園圍欄的影子被歪曲。

影，又稱影子、陰影，是從光源直射的光被不透明的物體阻擋而產生的黑暗範圍。當光源置於物體前方時，影會佔據物體背後三維空間的體積。 影的截面是一個二維的剪影，是物體阻擋光線的反向投影。

點光源與非點光源

本影、半影和偽本影

點光源只會投射出簡單的影，稱之為「本影」。對於非點光源或擴展光源產生的影，可以分為本影、半影和偽本影。寬大、分散的光源會投射出模糊的影。如果兩個半影重疊，影會看似互相吸引並合併在一起。這現象被稱為shadow blister effect。

陰影區域的框線可以藉由描繪擴展光源最外層所發射的光來界定。本影區不會從光源接收到任何光線，因此這區域是最暗的。在本影區裡無法直接看到光源的任何部分。相對的，半影會被部分光源照亮，呈現中等亮度。在半影區裡能夠看見光源，只是部分光源會被物體遮擋。

如果存在多個光源，各個影的重疊部分會變得更暗，形成不同亮度或顔色。漫射光會使影的框線變得不明顯，甚至消失。灰暗的天空中，閃電可以產生肉眼可見的影。由於在外太空的真空中不存在大氣漫射的效應，這裡產生的影在光暗交界處非常鮮明、對比度極高。

若以物體觸碰投射影子的表面（人站在地面上、插在土裡的柱子等），影會匯聚在物體與表面的接觸點上。

在不考慮失真的狀態下，影和從亮面觀察剪影的像是一致的。若從另一面觀察，影和像是呈現鏡像的。

天文學

三個衛星（木卫一、木卫二和木卫四）帶著它們的影子越過木星.

本影、半影和偽本影很常用來描述由天體形成的影，有時候也會用來描述黑暗的程度，比如説太陽黑子。视星等小於或等於-4的天體可以投射出人類肉眼可見的影。 能將可見的影投射在地球上的天體有太陽和月球。在適合的條件下，金星和木星也能把影投射在地球上。 行星面朝恆星的半球體遮擋陽光，造成了行星上的夜晚。

地球的影投射在月球上，稱為月食。相應的，月球的影投射在地球上，稱為日食。

白天的變化

The sun casts shadows that change dramatically through the day. The length of a shadow cast on the ground is proportional to the 三角函数 of the sun's 地平坐標系—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 阿拉伯沙漠.

傳播速度

蒸汽 phase 火山噴發類型 of Castle Geyser in 黃石國家公園 casts a shadow on its own steam. 雲隙光 are also visible.

The farther the distance from the object blocking the light to the surface of projection, the larger the silhouette (they are considered 比例). 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 光年s. The loss of light, which projects the shadow, will move towards the surface of projection at 光速.

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) 狭义相对论 between those distances with the shadow's edge.

顔色

Visual artists are usually very aware of colored light emitted or reflected from several sources, which can generate complex multicolored shadows. 明暗对照法, 晕涂法, and 剪影 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 瑞利散射, 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.

空間

Fog shadow of the south tower of the 金门大桥

A shadow occupies a three-dimensional volume of space, but this is usually not visible until it projects onto a reflective surface. A light 霧, 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 雲隙光 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.

反轉

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.

照相

月光 shadow of a photographer

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. 曝光 must be adjusted (unless special effects are wanted) to allow the 底片 or 传感器, which has limited 动态范围, to record detail in the highlights without them being washed out, and in the shadows without their becoming undifferentiated black areas.

On 卫星影像 and 空拍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.

類比概念

Shadow as a term is often used for any occlusion or blockage, not just those with respect to light. For example, a 雨影 is a dry area, which with respect to the prevailing 風部 direction, is beyond a 山脉; 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.

文化

An unattended Shade (mythology) 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 纹章学, 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 顏色 (紋章學) 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 and modern graphic novels, like shadows who come to life, are often evil beings trying to control the people they reflect. The film Upside-Down Magic (film) features an antagonistic shadow spirit who possesses people.

能量生產

Scientists from the 新加坡国立大学 presented a shadow-effect energy generator (SEG), which consists of cells of 金 (消歧义) deposited on a 晶圓 attached on a plastic film. The generator has a 功率密度 of 0.14 μW cm⁻² under indoor conditions (0.001 sun).

Gallery

• 
  Non-diffuse lighting in outer space causes deep shadows
• 
  Reversed text in shadow
• 
  蘇特洛塔 casts a 3D fog shadow
• 
  This photo of jasmine flowers has only soft shadows cast by diffused light
• 
  Clouds, and shadows over the 地中海
• 
  Shadow cast by 飛機雲 of passing aircraft
• 
  Shadow of a parent and child
• 
  Tree shadow
• 
  Shadow on the Castle
• 
  Long shadow of a dead tree with its branches on dry fields, late afternoon
• 
  When the sun is low, shadows become long, and details get the wrong proportions.

參見

• 黑滴現象
• 卷积, for more physical and mathematical discussion about shadows
• 地影
• 晷針
• shade (shadow)
• 影子内阁
• 阴影贴图, in computer 3D graphics
• 鬼影人
• 皮影戲
• Shadowgraphy (performing art) or ombromanie, the art of hand shadows
• Sciography, the art of architectural shadows
• 偽裝 (動物學)

外部連結

• Art project about shadows and their philosophical connotations
• How sun casts shadows over day hours

Category:Shadows