Blue is one of the three primary colours of pigments in painting and traditional colour theory, as well as in the RGB colour model. It lies between violet and green on the spectrum of visible light. The eye perceives blue when observing light with a dominant wavelength between approximately 450 and 495 nanometres. Most blues contain a slight mixture of other colors; azure contains some green, while ultramarine contains some violet. The clear daytime sky and the deep sea appear blue because of an optical effect known as Rayleigh scattering. An optical effect called Tyndall scattering explains blue eyes. Distant objects appear more blue because of another optical effect called atmospheric perspective.

Blue has been an important colour in art and decoration since ancient times. The semi-precious stone lapis lazuli was used in ancient Egypt for jewellery and ornament and later, in the Renaissance, to make the pigment ultramarine, the most expensive of all pigments. In the eighth century Chinese artists used cobalt blue to colour fine blue and white porcelain. In the Middle Ages, European artists used it in the windows of Cathedrals. Europeans wore clothing coloured with the vegetable dye woad until it was replaced by the finer indigo from America. In the 19th century, synthetic blue dyes and pigments gradually replaced mineral pigments and synthetic dyes. Dark blue became a common colour for military uniforms and later, in the late 20th century, for business suits. Because blue has commonly been associated with harmony, it was chosen as the colour of the flags of the United Nations and the European Union.

Surveys in the US and Europe show that blue is the colour most commonly associated with harmony, faithfulness, confidence, distance, infinity, the imagination, cold, and sometimes with sadness. In US and European public opinion polls it is the most popular colour, chosen by almost half of both men and women as their favourite colour. The same surveys also showed that blue was the colour most associated with the masculine, just ahead of black, and was also the colour most associated with intelligence, knowledge, calm and concentration.

Shades and variations
Blue is the colour of light between violet and green on the visible spectrum. Hues of blue include indigo and ultramarine, closer to violet; pure blue, without any mixture of other colours; Cyan, which is midway in the spectrum between blue and green, and the other blue-greens turquoise, teal, and aquamarine.

Blue also varies in shade or tint; darker shades of blue contain black or grey, while lighter tints contain white. Darker shades of blue include ultramarine, cobalt blue, navy blue, and Prussian blue; while lighter tints include sky blue, azure, and Egyptian blue. (For a more complete list see the List of colours).

Blue pigments were originally made from minerals such as lapis lazuli, cobalt and azurite, and blue dyes were made from plants; usually woad in Europe, and Indigofera tinctoria, or true indigo, in Asia and Africa. Today most blue pigments and dyes are made by a chemical process.

Optics
Human eyes perceive blue when observing light which has a dominant wavelength of roughly 450–495 nanometres. Blues with a higher frequency and thus a shorter wavelength gradually look more violet, while those with a lower frequency and a longer wavelength gradually appear more green. Pure blue, in the middle, has a wavelength of 470 nanometres.

Isaac Newton included blue as one of the seven colours in his first description the visible spectrum, He chose seven colours because that was the number of notes in the musical scale, which he believed was related to the optical spectrum. He included indigo, the hue between blue and violet, as one of the separate colours, though today it is usually considered a hue of blue.

In painting and traditional colour theory, blue is one of the three primary colours of pigments (red, yellow, blue), which can be mixed to form a wide gamut of colours. Red and blue mixed together form violet, blue and yellow together form green. Mixing all three primary colours together produces a dark grey. From the Renaissance onwards, painters used this system to create their colours. (See RYB colour system.)

The RYB model was used for colour printing by Jacob Christoph Le Blon as early as 1725. Later, printers discovered that more accurate colours could be created by using combinations of magenta, cyan, yellow and black ink, put onto separate inked plates and then overlaid one at a time onto paper. This method could produce almost all the colours in the spectrum with reasonable accuracy.

In the 19th century the Scottish physicist James Clerk Maxwell found a new way of explaining colours, by the wavelength of their light. He showed that white light could be created by combining red, blue and green light, and that virtually all colours could be made by different combinations of these three colours. His idea, called additive colour or the RGB colour model, is used today to create colours on televisions and computer screens. The screen is covered by tiny pixels, each with three fluorescent elements for creating red, green and blue light. If the red, blue and green elements all glow at once, the pixel looks white. As the screen is scanned from behind with electrons, each pixel creates its own designated colour, composing a complete picture on the screen.

On the HSV colour wheel, the complement of blue is yellow; that is, a colour corresponding to an equal mixture of red and green light. On a colour wheel based on traditional colour theory (RYB) where blue was considered a primary colour, its complementary colour is considered to be orange (based on the Munsell colour wheel).

Pigments and dyes

Blue pigments were made from minerals, especially lapis lazuli and azurite (Cu₃(CO₃)₂(OH)₂). These minerals were crushed, ground into powder, and then mixed with a quick-drying binding agent, such as egg yolk (tempera painting); or with a slow-drying oil, such as linseed oil, for oil painting. To make blue stained glass, cobalt blue (cobalt(II) aluminate: CoAl₂O₄)pigment was mixed with the glass. Other common blue pigments made from minerals are ultramarine (Na_8-10Al₆Si₆O₂₄S_2-4), cerulean blue (primarily cobalt (II) stanate: Co₂SnO₄), and Prussian blue (milori blue: primarily Fe₇(CN)₁₈).

Natural dyes to colour cloth and tapestries were made from plants. Woad and true indigo were used to produce indigo dye used to colour fabrics blue or indigo. Since the 18th century, natural blue dyes have largely been replaced by synthetic dyes.

Lapis lazuli, mined in Afghanistan for more than three thousand years, was used for jewellery and ornaments, and later was crushed and powdered and used as a pigment. The more it was ground, the lighter the blue colour became.

Azurite, common in Europe and Asia, is produced by the weathering of copper ore deposits. It was crushed and powdered and used as a pigment from ancient times,

Natural ultramarine, made by grinding and purifying lapis lazuli, was the finest available blue pigment in the Middle Ages and the Renaissance. It was extremely expensive, and in Italian Renaissance art, it was often reserved the robes of the Virgin Mary.

Egyptian blue, the first artificial pigment, created in the third millennium BC in Ancient Egypt by grinding sand, copper and natron, and then heating them. It was often used in tomb paintings and funereal objects to protect the dead in their afterlife.

Ground azurite was often in Renaissance used as a substitute for the much more expensive lapis lazuli. It made a rich blue, but was unstable and could turn dark green over time.

Cerulean was created with copper and cobalt oxide, and used to make a sky blue colour. Like azurite, it could fade or turn green.

Cobalt blue. Cobalt has been used for centuries to colour glass and ceramics; it was used to make the deep blue stained glass windows of Gothic cathedrals and Chinese porcelain beginning in the T’ang Dynasty. In 1799 a French chemist, Louis Jacques Thénard, made a synthetic cobalt blue pigment which became immensely popular with painters.

Indigo dye is made from the woad, Indigofera tinctoria, a plant common in Asia and Africa but little known in Europe until the 15th century. Its importation into Europe revolutionised the colour of clothing. It also became the colour used in blue denim and jeans. Nearly all indigo dye produced today is synthetic.

Chemical structure of indigo dye, a widely produced blue dye. Blue jeans consist of 1–3% by weight of this organic compound.

Synthetic ultramarine pigment, invented in 1826, has the same chemical composition as natural ultramarine. It is more vivid than natural ultramarine because the particles are smaller and more uniform in size, and thus distribute the light more evenly.

Scientific natural standards
Emission spectrum of Cu²⁺

Electronic spectrum of aqua-ions Cu(H₂O)²⁺₆

Why the sky and sea appear blue
Of the colours in the visible spectrum of light, blue has a very short wavelength, while red has the longest wavelength. When sunlight passes through the atmosphere, the blue wavelengths are scattered more widely by the oxygen and nitrogen molecules, and more blue comes to our eyes. This effect is called Rayleigh scattering, after Lord Rayleigh, the British physicist who discovered it. It was confirmed by Albert Einstein in 1911.

Near sunrise and sunset, most of the light we see comes in nearly tangent to the Earth’s surface, so that the light’s path through the atmosphere is so long that much of the blue and even green light is scattered out, leaving the sun rays and the clouds it illuminates red. Therefore, when looking at the sunset and sunrise, the colour red is more perceptible than any of the other colours.

The sea is seen as blue for largely the same reason: the water absorbs the longer wavelengths of red and reflects and scatters the blue, which comes to the eye of the viewer. The colour of the sea is also affected by the colour of the sky, reflected by particles in the water; and by algae and plant life in the water, which can make it look green; or by sediment, which can make it look brown.

Atmospheric perspective
The farther away an object is, the more blue it often appears to the eye. For example, mountains in the distance often appear blue. This is the effect of atmospheric perspective; the farther an object is away from the viewer, the less contrast there is between the object and its background colour, which is usually blue. In a painting where different parts of the composition are blue, green and red, the blue will appear to be more distant, and the red closer to the viewer. The cooler a colour is, the more distant it seems.

Astronomy
•A blue giant is the largest type of stars. A blue supergiant is even bigger.

Blue eyes
Blue eyes do not actually contain any blue pigment. Eye colour is determined by two factors: the pigmentation of the eye’s iris and the scattering of light by the turbid medium in the stroma of the iris. In humans, the pigmentation of the iris varies from light brown to black. The appearance of blue, green, and hazel eyes results from the Rayleigh scattering of light in the stroma, an optical effect similar to what accounts for the blueness of the sky. The irises of the eyes of people with blue eyes contain less dark melanin than those of people with brown eyes, which means that they absorb less short-wavelength blue light, which is instead reflected out to the viewer. Eye colour also varies depending on the lighting conditions, especially for lighter-coloured eyes.

Blue eyes are most common in Ireland, the Baltic Sea area and Northern Europe, and are also found in Eastern, Central, and Southern Europe. Blue eyes are also found in parts of Western Asia, most notably in Afghanistan, Syria, Iraq, and Iran. In Estonia, 99% of people have blue eyes. In Denmark 30 years ago, only 8% of the population had brown eyes, though through immigration, today that number is about 11%. In Germany, about 75% have blue eyes.

In the United States, as of 2006, one out of every six people, or 16.6% of the total population, and 22.3% of the white population, have blue eyes, compared with about half of Americans born in 1900, and a third of Americans born in 1950. Blue eyes are becoming less common among American children. In the US, boys are 3–5 per cent more likely to have blue eyes than girls.

Lasers
Lasers emitting in the blue region of the spectrum became widely available to the public in 2010 with the release of inexpensive high-powered 445-447 nm laser diode technology. Previously the blue wavelengths were accessible only through DPSS which are comparatively expensive and inefficient, however these technologies are still widely used by the scientific community for applications including optogenetics, Raman spectroscopy, and particle image velocimetry, due to their superior beam quality. Blue gas lasers are also still commonly used for holography, DNA sequencing, optical pumping, and other scientific and medical applications.

Source From Wikipedia