In ancient Egypt and ancient Rome, priestesses wore white as a symbol of purity, and Romans wore a white toga as a symbol of citizenship. In the Middle Ages and Renaissance a white unicorn symbolized chastity, and a white lamb sacrifice and purity. It was the royal color of the Kings of France, and of the monarchist movement that opposed the Bolsheviks during the Russian Civil War (1917–1922). Greek and Roman temples were faced with white marble, and beginning in the 18th century, with the advent of neoclassical architecture, white became the most common color of new churches, capitols and other government buildings, especially in the United States. It was also widely used in 20th century modern architecture as a symbol of modernity and simplicity.
According to surveys in Europe and the United States, white is the color most often associated with perfection, the good, honesty, cleanliness, the beginning, the new, neutrality, and exactitude. White is an important color for almost all world religions. The Pope, the head of the Roman Catholic Church, has worn white since 1566, as a symbol of purity and sacrifice. In Islam, and in the Shinto religion of Japan, it is worn by pilgrims; and by the Brahmins in India. In Western cultures and in Japan, white is the most common color for wedding dresses, symbolizing purity and virginity. In many Asian cultures, white is also the color of mourning.
Etymology
The word white continues Old English hwīt, ultimately from a Common Germanic *χwītaz also reflected in OHG (h)wîz, ON hvítr, Goth. ƕeits. The root is ultimately from Proto-Indo-European language *kwid-, surviving also in Sanskrit śveta "to be white or bright" and Slavonic světŭ "light". The Icelandic word for white, hvítur, is directly derived from the Old Norse form of the word hvítr. Common Germanic also had the word *blankaz ("white, bright, blinding"), borrowed into Late Latin as *blancus, which provided the source for Romance words for "white" (Catalan, Occitan and French blanc, Spanish blanco, Italian bianco, Galician-Portuguese branco, etc.). The antonym of white is black.
Some non-European languages have a wide variety of terms for white. The Inuit language has seven different words for seven different nuances of white. Sanskrit has specific words for bright white, the white of teeth, the white of sandalwood, the white of the autumn moon, the white of silver, the white of cow's milk, the white of pearls, the white of a ray of sunlight, and the white of stars. Japanese has six different words, depending upon brilliance or dullness, or if the color is inert or dynamic.
Scientific understanding (Color science)
Light is perceived by the human visual system as white when the incoming light to the eye stimulates all three types of color sensitive cone cells in the eye in roughly equal amounts. Materials that do not emit light themselves appear white if their surfaces reflect back most of the light that strikes them in a diffuse way.
White light
In 1666, Isaac Newton demonstrated that white light could be broken up into its composite colors by passing it through a prism, then using a second prism to reassemble them. Before Newton, most scientists believed that white was the fundamental color of light.
White light can be generated by the sun, by stars, or by earthbound sources such as fluorescent lamps, white LEDs and incandescent bulbs. On the screen of a color television or computer, white is produced by mixing the primary colors of light: red, green and blue (RGB) at full intensity, a process called additive mixing (see image below). White light can be fabricated using light with only two wavelengths, for instance by mixing light from a red and cyan laser or yellow and blue lasers. This light will however have very few practical applications since color rendering of objects will be greatly distorted.
The fact that light sources with vastly different spectral power distributions can result in a similar sensory experience is due to the way the light is processed by the visual system. One color that arise from two different spectral power distributions is called a metamerism.
Many of the light sources that emit white light emit light at almost all visible wavelengths (sun light, incandescent lamps of various Color temperatures). This has led to the notion that white light can be defined as a mixture of "all colors" or "all visible wavelengths". This widespread idea is a misconception, and might originally stem from the fact that Newton discovered that sunlight is composed of light with wavelengths across the visible spectrum. Concluding that since "all colors" produce white light then white must be made up of "all colors" is a common logical error called affirming the consequent, which might be the cause of the misunderstanding.
A range of spectral distributions of light sources can be perceived as white—there is no single, unique specification of "white light". For example, when you buy a "white" light bulb, you might buy one labeled 2700K, 6000K, etc., which produce light having very different spectral distributions, and yet this will not prevent you from identifying the color of objects that they illuminate.
White objects
Color vision allows us to distinguish different objects by their color. In order to do so, color constancy can keep the perceived color of an object relatively unchanged when the illumination changes among various broad (whitish) spectral distributions of light.
The same principle is used in photography and cinematography where the choice of white point determines a transformation of all other color stimuli. Changes in or manipulation of the white point can be used to explain some optical illusions such as The dress.
While there is no single, unique specification of "white light", there is indeed a unique specification of "white object", or, more specifically, "white surface". A perfectly white surface diffusely reflects (scatters) all visible light that strikes it, without absorbing any, irrespective of the light's wavelength or spectral distribution. Since it does not absorb any of the incident light, white is the lightest possible color. If the reflection is not diffuse but rather specular, this describes a mirror rather than a white surface.
Reflection of 100% of incident light at all wavelengths is a form of uniform reflectance, so white is an achromatic color, meaning a color without hue. The color stimulus produced by the perfect diffuser is usually considered to be an achromatic stimulus for all illuminants, except for those whose light sources appear to be highly chromatic.
Color constancy is achieved by chromatic adaptation. The International Commission on Illumination defines white (adapted) as "a color stimulus that an observer who is [chromatically] adapted to the viewing environment would judge to be perfectly achromatic and to have a luminance factor of unity. The color stimulus that is considered to be the adapted white may be different at different locations within a scene.
Why snow, clouds and beaches are white
Beaches with sand containing high amounts of quartz or eroded limestone also appear white, since quartz and limestone reflect or scatter sunlight, rather than absorbing it. Tropical white sand beaches may also have a high quantity of white calcium carbonate from tiny bits of seashells ground to fine sand by the action of the waves.
Snow is a mixture of air and tiny ice crystals. When white sunlight enters snow, very little of the spectrum is absorbed; almost all of the light is reflected or scattered by the air and water molecules, so the snow appears to be the color of sunlight, white. Sometimes the light bounces around inside the ice crystals before being scattered, making the snow seem to sparkle.
In the case of glaciers, the ice is more tightly pressed together and contains little air. As sunlight enters the ice, more light of the red spectrum is absorbed, so the light scattered will be bluish.
Clouds are white for the same reason as ice. They are composed of water droplets or ice crystals mixed with air, very little light that strikes them is absorbed, and most of the light is scattered, appearing to the eye as white. Shadows of other clouds above can make clouds look gray, and some clouds have their own shadow on the bottom of the cloud.
Many mountains with winter or year-round snow cover are named accordingly: Mauna Kea means white mountain in Hawaiian, Mont Blanc means white mountain in French. Changbai Mountains literally meaning perpetually white mountains, marks the border between China and Korea.
White materials
Chalk is a kind of limestone, made of the mineral calcite, or calcium carbonate. It was originally deposited under the sea as the scales or plates of tiny micro-organisms called Coccolithophore. It was the first white pigment used by prehistoric artists in cave paintings. The chalk used on blackboards today is usually made of gypsum or calcium sulphate, a powder pressed into sticks.
Bianco di San Giovanni is a pigment used in the Renaissance, which was described by the painter Cennino Cennini in the 15th century. It is similar to chalk, made of calcium carbonate with calcium hydroxide. It was made of dried lime which was made into a powder, then soaked in water for eight days, with the water changed each day. It was then made into cakes and dried in the sun.
Lead white was being produced during the 4th century BC; the process is described is Pliny the Elder, Vitruvius and the ancient Greek author Theophrastus. Pieces of lead were put into clay pots which had a separate compartment filled with vinegar. The pots in turn were piled on shelves close to cow dung. The combined fumes of the vinegar and the cow dung caused the lead to corrode into lead carbonate. It was a slow process which could take a month or more. It made an excellent white and was used by artists for centuries, but it was also toxic. It was replaced in the 19th century by zinc white and titanium white.
Titanium white is the most popular white for artists today; it is the brightest available white pigment, and has twice the coverage of lead white. It first became commercially available in 1921. It is made out of titanium dioxide, from the minerals brookite, anatase, rutile, or ilmenite, currently the major source. Because of its brilliant whiteness, it is used as a colorant for most toothpaste and sunscreen.
Zinc white is made from zinc oxide. It is similar to but not as opaque as titanium white. It is added to some breakfast cereals, since zinc is an important nutrient. Chinese white is a variety of zinc white made for artists.
Some materials can be made to look "whiter than white", this is achieved using optical brightener agents (OBA). These are chemical compounds that absorb light in the ultraviolet and violet region (usually 340–370 nm) of the electromagnetic spectrum, and re-emit light in the blue region (typically 420–470 nm). OBAs are often used in paper and clothing to create an impression of very bright white. This is due to the fact that the materials actually send out more visible light than they receive.
Bleach and bleaching
Bleaching is a process for whitening fabrics which has been practiced for thousands of years. Sometimes it was simply a matter of leaving the fabric in the sun, to be faded by the bright light. In the 18th century several scientists developed varieties of chlorine bleach, including sodium hypochlorite and calcium hypochlorite (bleaching powder). Bleaching agents that do not contain chlorine most often are based on peroxides, such as hydrogen peroxide, sodium percarbonate and sodium perborate. While most bleaches are oxidizing agents, a fewer number are reducing agents such as sodium dithionite.
Bleaches attack the chromophores, the part of a molecule which absorbs light and causes fabrics to have different colors. An oxidizing bleach works by breaking the chemical bonds that make up the chromophore. This changes the molecule into a different substance that either does not contain a chromophore, or contains a chromophore that does not absorb visible light. A reducing bleach works by converting double bonds in the chromophore into single bonds. This eliminates the ability of the chromophore to absorb visible light.
Sunlight acts as a bleach through a similar process. High energy photons of light, often in the violet or ultraviolet range, can disrupt the bonds in the chromophore, rendering the resulting substance colorless.
Some detergents go one step further; they contain fluorescent chemicals which glow, making the fabric look literally whiter than white.
In the natural world
Astronomy
A white dwarf is a stellar remnant composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy. A white dwarf is very hot when it is formed, but since it has no source of energy, it will gradually radiate away its energy and cool down. This means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very long time, a white dwarf will cool to temperatures at which it will no longer emit significant heat or light, and it will become a cold black dwarf. However, since no white dwarf can be older than the age of the Universe (approximately 13.8 billion years), even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins, and no black dwarfs are thought to exist yet.
An A-type main-sequence star (A V) or A dwarf star is a main-sequence (hydrogen-burning) star of spectral type A and luminosity class V. These stars have spectra which are defined by strong hydrogen Balmer absorption lines. They have masses from 1.4 to 2.1 times the mass of the Sun and surface temperatures between 7600 and 11 500 K.
Biology and ecology
In nature, snow and clouds appear white because they are composed of water droplets or ice crystals mixed with air; when white sunlight enters snow, very little of the spectrum is absorbed; almost all of the light is reflected or scattered by the air and water molecules, so the snow appears to be the color of sunlight, white.Most white animals have their color as a form of camouflage in winter.
Source From Wikipedia
没有评论:
发表评论