Blue facts for kids

Mountain Bluebird

The famous Blue Marble picture of Earth, taken by NASA's Apollo 17

Blue is one of the colors of the rainbow that people can see. It is one of the three primary colors, along with red and yellow. It has the shortest wavelength of these colors (about 470 nanometers).

Blue is the color of the Earth's sky and sea. Earth looks blue when seen from outer space by astronauts.

Blue is a color used to show coldness. It is also one of the colors your lips can turn if you are chilly.

Blue is the color of a bluebird. A blue jay is the color blue.

Comparison of blue, indigo, violet, and purple

Tones of blue color comparison chart

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 on the spectrum between blue and green, and the other blue-greens turquoise, teal, and aquamarine.

Blues also vary 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Blue pigments were originally made from minerals such as lapis lazuli, cobalt and azurite, and blue dyes were made from plants; usually woad in Europe, and Indigofera tinctoria, or true indigo,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.

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  Navy blue, here worn by Admiral Horatio Nelson, is the darkest shade of pure blue.

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  Sky blue or pale azure, mid-way on the RBG colour wheel between blue and cyan.

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  Egyptian blue goblet from Mesopotamia, 1500–1300 BC. This was the first synthetic blue, first made in about 2500 BC.

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  Extract of natural indigo, the most popular blue dye before the invention of synthetic indigo.

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  A block of lapis lazuli, originally used to make ultramarine.

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  Ultramarine, slightly violet-blue, in a painting by Giovanni Bellini. It was the most expensive pigment of Renaissance.

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  Cobalt coloured the stained glass windows of Sainte-Chapelle in Paris (1250)

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  Prussian blue, the colour of the uniforms of the army of Prussia, was invented in about 1706

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  Vincent Van Gogh used the synthetic pigment cobalt blue for the sky of his starry night.

In science and industry

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.

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  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.

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  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,

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  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.

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  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.

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

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  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.

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  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.

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  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.

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  A new synthetic blue created in the 1930s is phthalocyanine, an intense colour widely used for making blue ink, dye, and pigment.

Optics

Colour	Frequency	Wavelength
violet	668–789 THz	380–450 nm
blue	606–668 THz	450–495 nm
green	526–606 THz	495–570 nm
yellow	508–526 THz	570–590 nm
orange	484–508 THz	590–620 nm
red	400–484 THz	620–750 nm

Human eyes perceive blue when observing light which has a wavelength between 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.

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  Additive colour mixing. The projection of primary colour lights on a screen shows secondary colours where two overlap; the combination red, green, and blue each in full intensity makes white.

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  Blue and orange pixels on an LCD television screen. Closeup of the red, green and blue sub-pixels on left.

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).

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.

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  Blue light is scattered more than other wavelengths by the gases in the atmosphere, giving the Earth a blue halo when seen from space.

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  An example of aerial, or atmospheric perspective. Objects become more blue and lighter in colour the farther they are from the viewer, because of Rayleigh scattering.

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  Under the sea, red and other light with longer wavelengths is absorbed, so white objects appear blue. The deeper you go, the darker the blue becomes. In the open sea, only about one per cent of light penetrates to a depth of 200 metres. (See underwater and euphotic depth)

Blue eyes

Blue eyes actually contain no blue pigment. The colour is caused by an effect called Rayleigh scattering, which also makes the sky appear blue.

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.

In nature

Blue compounds are relatively rare in nature.

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  Lactarius indigo, or the blue milk mushroom

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  Cornflower

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  Myosotis, or forget-me-not

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  Blue seeds of the Ravenala tree from Madagascar

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  The Morpho peleides butterfly. The blue is caused by iridescence, the diffraction of light from millions of tiny scales on the wings. The colour is intended to frighten predators.

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  River kingfisher

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  Linckia blue starfish

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  Blue sapphire, a gemstone of the mineral corundum. Trace amounts of iron colour it blue; if there are traces of chromium instead, it has a red tint and is called a ruby.

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  Dried crystals of copper sulphate

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  Blueberries

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  Dendrobates azureus, the poison dart frog from Brazil. Its skin contains alkaloids, which can paralyse or kill predators.

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  Blue jay

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  A blue whale, the largest known animal to have ever existed, seen from above. The back is a pale blue grey.

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  Blue carpet of Lupinus pilosus.

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  Blue Anemone coronaria.

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  The body feathers of Pavo cristatus (peacock) are coloured with shining blue.

Animals

• When an animal's coat is described as "blue", it usually refers to a shade of grey that takes on a bluish tint, a diluted variant of a pure black coat. This designation is used for a variety of animals, including dog coats, some rat coats, cat coats, some chicken breeds, some horse coat colours and rabbit coat colours. Some animals, such as giraffes and lizards, also have blue tongues.

Related pages

• List of colors
• Aqua
• Azure
• Blue-green
• Blue-violet
• Cerulean
• Cyan
• Electric blue
• Indigo
• Navy blue
• Prussian blue
• Teal blue

Images for kids

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  Various shades of blue

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  Blue eyes actually contain no blue pigment. The colour is caused by an effect called Tyndall scattering.

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  Red, green, and blue subpixels on an LCD display.

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  Egyptian blue

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  YInMn blue.

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  Prussian blue, FeIII
  4[FeII
  (CN)
  6]
  3, is the blue of blueprints.

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  The Blue Boy (1770), featuring lapis lazuli, indigo, and cobalt colourants,

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  The Great Wave off Kanagawa illustrates the use of Prussian blue

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  A synthetic indigo dye factory in Germany in 1890.

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  Logan sapphire

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  Primula acaulis

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  Morning glory (Ipomoea acuminata)

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  Vaccinium corymbosum

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  Blue Delphinium flower

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  Morpho butterfly

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  Indigo buntings have iridescent feathers.

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  Blue facial ridges of mandrill

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  Blue poison dart frog

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  The mandarin fish is one of few animal species with blue pigment

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  Lapis lazuli bowl from Iran, end of 3rd – beginning of 2nd millennium BC (Louvre Museum)

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  Stained glass window at Saint Denis Basilica (1130-1140), coloured with cobalt blue

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  Detail of the Blue Virgin Window, Chartres Cathedral (12th c.)

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  The Wilton Diptych (1395–1399). The Virgin Mary was traditionally shown in blue(14th c.)

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  Ming Dynasty, Porcelain vase painted with cobalt blue under transparent glaze. (15th c.) (Metropolitan Museum)

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  Chinese porcelain flasks with blue dragon design (15th century)

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  Delftware plaque with cobalt blue painting (1683) (Rijksmuseum, Amsterdam)

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  Urn by Josiah Wedgewood (1780s) (Metropolitan Museum)

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  Beau Brummel (1776-1840) introduced the ancestor of the modern blue suit

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  A California gold miner in blue jeans (1853)

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  New York City police in 1871

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  Blue conveys melancholy in Picasso's The Old Guitarist (1903-1904).

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  "The Conversation" by Henri Matisse (1908-1912)

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  Officers of the London Metropolitan Police

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  Sailors of the Royal Navy

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  Ukrainian police officer in Donetsk

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  Icon of Virgin Mary, Russia (circa 600 AD)

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  Blue domes of the Church dedicated to St. Spirou in Firostefani, Santorini island (Thira), Greece.

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  Blue mosque in Herat, Afghanistan (Mosaic from 15th C.)

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  The flag of Israel uses a special variety of blue, called tekhelet

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  The Italian national football team

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  Serbian national volleyball team, 2012 Olympics

See also

In Spanish: Azul para niños