Yellow color in science and nature

Yellow is the color between green and orange on the spectrum of visible light. It is evoked by light with a dominant wavelength of roughly 570–590 nm. It is a primary color in subtractive color systems, used in painting or color printing. In the RGB color model, used to create colors on television and computer screens, yellow is a secondary color made by combining red and green at equal intensity. Carotenoids give the characteristic yellow color to autumn leaves, corn, canaries, daffodils, and lemons, as well as egg yolks, buttercups, and bananas. They absorb light energy and protect plants from photodamage. Sunlight has a slight yellowish hue, due to the surface temperature of the sun.

Because it was widely available, yellow ochre pigment was one of the first colors used in art; the Lascaux cave in France has a painting of a yellow horse 17,000 years old. Ochre and orpiment pigments were used to represent gold and skin color in Egyptian tombs, then in the murals in Roman villas. In the early Christian church, yellow was the color associated with the Pope and the golden keys of the Kingdom, but was also associated with Judas Iscariot and was used to mark heretics. In the 20th century, Jews in Nazi-occupied Europe were forced to wear a yellow star. In China, yellow was the color of the Middle Kingdom, and could be worn only by the Emperor and his household; special guests were welcomed on a yellow carpet.

According to surveys in Europe, Canada, and the United States, yellow is the color people most often associate with amusement, gentleness, humor, and spontaneity, but also with duplicity, envy, jealousy, avarice, and, in the U.S., with cowardice. In Iran it has connotations of pallor/sickness, but also wisdom and connection. In China and many Asian countries, it is seen as the color of happiness, glory, harmony and wisdom.

Etymology

The word yellow comes from the Old English geolu, geolwe (oblique case), meaning "yellow, yellowish", derived from the Proto-Germanic word gelwaz "yellow". It has the same Indo-European base, gʰel-, as the words gold and yell; gʰel- means both bright and gleaming, and to cry out.

The English term is related to other Germanic words for yellow, namely Scots yella, East Frisian jeel, West Frisian giel, Dutch geel, German gelb, and Swedish and Norwegian gul. According to the Oxford English Dictionary, the oldest known use of this word in English is from The Epinal Glossary in 700.

Science and nature

Optics, color printing, and computer screens

Yellow is found between green and orange on the spectrum of visible light. It is the color the human eye sees when it looks at light with a dominant wavelength between 570 and 590 nanometers.

In color printing, yellow is one of the three colors of ink, along with magenta and cyan, which, along with black, can be overlaid in the right combination, along with black, to print any full color image. (See the CMYK color model). A particular yellow is used, called Process yellow (also known as "pigment yellow", "printer's yellow", and "canary yellow") subtractive primary colors, along with magenta and cyan. Process yellow is not an RGB color, and there is no fixed conversion from CMYK primaries to RGB. Different formulations are used for printer's ink, so there can be variations in the printed color that is pure yellow ink.

The yellow on a color television or computer screen is created in a completely different way; by combining green and red light at the right level of intensity. (See RGB color model).

Complementary colors

Traditionally, the complementary color of yellow is purple; the two colors are opposite each other on the color wheel long used by painters. Vincent Van Gogh, an avid student of color theory, used combinations of yellow and purple in several of his paintings for the maximum contrast and harmony.

Hunt defines that "two colors are complementary when it is possible to reproduce the tristimulus values of a specified achromatic stimulus by an additive mixture of these two stimuli." That is, when two colored lights can be mixed to match a specified white (achromatic, non-colored) light, the colors of those two lights are complementary. This definition, however, does not constrain what version of white will be specified. In the nineteenth century, the scientists Grassmann and Helmholtz did experiments in which they concluded that finding a good complement for spectral yellow was difficult, but that the result was indigo, that is, a wavelength that today's color scientists would call violet or purple. Helmholtz says "Yellow and indigo blue" are complements. Grassmann reconstructs Newton's category boundaries in terms of wavelengths and says "This indigo therefore falls within the limits of color between which, according to Helmholtz, the complementary colors of yellow lie."

Newton's own color circle has yellow directly opposite the boundary between indigo and violet. These results, that the complement of yellow is a wavelength shorter than 450 nm, are derivable from the modern CIE 1931 system of colorimetry if it is assumed that the yellow is about 580 nm or shorter wavelength, and the specified white is the color of a blackbody radiator of temperature 2800 K or lower (that is, the white of an ordinary incandescent light bulb). More typically, with a daylight-colored or around 5000 to 6000 K white, the complement of yellow will be in the blue wavelength range, which is the standard modern answer for the complement of yellow.

Because of the characteristics of paint pigments and use of different color wheels, painters traditionally regard the complement of yellow as the color indigo or blue-violet.

Lasers

Lasers emitting in the yellow part of the spectrum are less common and more expensive than most other colors. In commercial products diode pumped solid state (DPSS) technology is employed to create the yellow light. An infrared laser diode at 808 nm is used to pump a crystal of neodymium-doped yttrium vanadium oxide (Nd:YVO4) or neodymium-doped yttrium aluminium garnet (Nd:YAG) and induces it to emit at two frequencies (281.76 THz and 223.39 THz: 1064 nm and 1342 nm wavelengths) simultaneously. This deeper infrared light is then passed through another crystal containing potassium, titanium and phosphorus (KTP), whose non-linear properties generate light at a frequency that is the sum of the two incident beams (505.15 THz); in this case corresponding to the wavelength of 593.5 nm ("yellow"). This wavelength is also available, though even more rarely, from a helium–neon laser. However, this not a true yellow, as it exceeds 590 nm. A variant of this same DPSS technology using slightly different starting frequencies was made available in 2010, producing a wavelength of 589 nm, which is considered a true yellow color. The use of yellow lasers at 589 nm and 594 nm have recently become more widespread thanks to the field of optogenetics.

Astronomy

Stars of spectral classes F and G, such as our Sun, have color temperatures that make them look "yellowish". The first astronomer to classify stars according to their color was F. G. W. Struve in 1827. One of his classifications was flavae, or yellow, and this roughly corresponded to stars in the modern spectral range F5 to K0. The Strömgren photometric system for stellar classification includes a 'y' or yellow filter that is centered at a wavelength of 550 nm and has a bandwidth of 20–30 nm.

Biology

Autumn leaves, yellow flowers, bananas, oranges and other yellow fruits all contain carotenoids, yellow and red organic pigments that are found in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria and some fungi. They serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they protect the green chlorophyll from photodamage.

In late summer, as daylight hours shorten and temperatures cool, the veins that carry fluids into and out of the leaf are gradually closed off. The water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. It is during this time that the chlorophyll begins to decrease. As the chlorophyll diminishes, the yellow and red carotenoids become more and more visible, creating the classic autumn leaf color.

Carotenoids are common in many living things; they give the characteristic color to carrots, corn, daffodils, rutabagas, buttercups and bananas. They are responsible for the red of cooked lobsters, the pink of flamingoes and salmon and the yellow of canaries and egg yolks.

Xanthophylls are the most common yellow pigments that form one of two major divisions of the carotenoid group. The name is from Greek xanthos (ξανθος, "yellow") + phyllon (φύλλον, "leaf"). Xanthophylls are most commonly found in the leaves of green plants, but they also find their way into animals through the food they eat. For example, the yellow color of chicken egg yolks, fat, and skin comes from the feed the chickens consume. Chicken farmers understand this, and often add xanthophylls, usually lutein, to make the egg yolks more yellow.

Bananas are green when they are picked because of the chlorophyll their skin contains. Once picked, they begin to ripen; hormones in the bananas convert amino acids into ethylene gas, which stimulates the production of several enzymes. These enzymes start to change the color, texture and flavor of the banana. The green chlorophyll supply is stopped and the yellow color of the carotenoids replaces it; eventually, as the enzymes continue their work, the cell walls break down and the bananas turn brown.

Fish

Yellowtail is the common name for dozens of different fish species that have yellow tails or a yellow body.

Yellowfin tuna (Thunnus albacares) is a species of tuna, having bright yellow anal and second dorsal fins. Found in tropical and subtropical seas and weighing up to 200 kg (440 lb), it is caught as a replacement for depleted stocks of bluefin tuna.

Insects

The yellow-fever mosquito (Aedes aegypti) is a mosquito so named because it transmits dengue fever and yellow fever, the mosquito-borne viruses.

Yellowjackets are black-and-yellow wasps of the genus Vespula or Dolichovespula (though some can be black-and-white, the most notable of these being the bald-faced hornet, Dolichovespula maculata). They can be identified by their distinctive black-and-yellow color, small size (slightly larger than a bee), and entirely black antennae.

Trees

Populus tremuloides is a deciduous tree native to cooler areas of North America, one of several species referred to by the common name aspen. Populus tremuloides is the most widely distributed tree in North America, being found from Canada to central Mexico.

The yellow birch (Betula alleghaniensis) is a birch species native to eastern North America, from Nova Scotia, New Brunswick, and southern Quebec west to Minnesota, and south in the Appalachian Mountains to northern Georgia. They are medium-sized deciduous trees and can reach about 20 m (66 ft) tall, trunks up to 80 cm (31 in) in diameter. The bark is smooth and yellow-bronze, and the wood is extensively used for flooring, cabinetry, and toothpicks.

The thorny yellowwood is an Australian rainforest tree which has deep yellow wood.

Yellow poplar is a common name for Liriodendron, the tuliptree. The common name is inaccurate as this genus is not related to poplars.

The Handroanthus albus is a tree with yellow flowers native to the Cerrado of Brazil.

Fruits, vegetables, and eggs

Many fruits are yellow when ripe, such as lemons and bananas, their color derived from carotenoid pigments. Egg yolks gain their color from xanthophylls, also a type of carotenoid pigment.

Flowers

Yellow is a common color of flowers.

Other plants

Rapeseed (Brassica napus), also known as rape or oilseed rape, is a bright yellow flowering member of the family Brassicaceae (mustard or cabbage family).

Goldenrod is a yellow flowering plant in the family Asteraceae.

Minerals and chemistry

Yellowcake (also known as urania and uranic oxide) is concentrated uranium oxide, obtained through the milling of uranium ore. Yellowcake is used in the preparation of fuel for nuclear reactors and in uranium enrichment, one of the essential steps for creating nuclear weapons.

Titan yellow (also known as clayton yellow), chemical formula C28H19Na2O6S4 has been used to determine magnesium in serum and urine, but the method is prone to interference, making the ammonium phosphate method superior when analysing blood cells, food or fecal material.

Methyl yellow (p-Dimethylaminoazobenzene) is a pH indicator used to determine acidity. It changes from yellow at pH=4.0 to red at pH=2.9.

Yellow fireworks are produced by adding sodium compounds to the firework mixture. Sodium has a strong emission at 589.3 nm (D-line), a very slightly orange-tinted yellow.

Amongst the elements, sulfur and gold are most obviously yellow. Phosphorus, arsenic and antimony have allotropes which are yellow or whitish-yellow; fluorine and chlorine are pale yellowish gases.

Pigments

Yellow ochre (also known as Mars yellow, Pigment yellow 42, 43), hydrated ferric oxide (Fe2O3.H2O), is a naturally occurring pigment found in clays in many parts of the world. It is non-toxic and has been used in painting since prehistoric times.

Indian yellow is a transparent, fluorescent pigment used in oil paintings and watercolors. Originally magnesium euxanthate, it was claimed to have been produced from the urine of Indian cows fed only on mango leaves. It has now been replaced by synthetic Indian yellow hue.

Naples Yellow (lead antimonate yellow) is one of the oldest synthetic pigments, derived from the mineral bindheimite and used extensively up to the 20th century. It is toxic and nowadays is replaced in paint by a mixture of modern pigments.

Cadmium Yellow (cadmium sulfide, CdS) has been used in artists' paints since the mid-19th century. Because of its toxicity, it may nowadays be replaced by azo pigments.

Chrome Yellow (lead chromate, PbCrO4), derived from the mineral crocoite, was used by artists in the earlier part of the 19th century, but has been largely replaced by other yellow pigments because of the toxicity of lead.

Zinc yellow or zinc chromate is a synthetic pigment made in the 19th century, and used by the painter Georges Seurat in his pointilist paintings. He did not know that it was highly unstable, and would quickly turn brown.

Titanium Yellow (nickel antimony titanium yellow rutile, NiO.Sb2O5.20TiO2) is created by adding small amounts of the oxides of nickel and antimony to titanium dioxide and heating. It is used to produce yellow paints with good white coverage and has the LBNL paint code "Y10".

Gamboge is an orange-brown resin, derived from trees of the genus Garcinia, which becomes yellow when powdered. It was used as a watercolor pigment in the far east from the 8th century – the name "gamboge" is derived from "Cambodia" – and has been used in Europe since the 17th century.

Orpiment, also called King's Yellow or Chinese Yellow is arsenic trisulfide (As2S3) and was used as a paint pigment until the 19th century when, because of its high toxicity and reaction with lead-based pigments, it was generally replaced by Cadmium Yellow.

azo dye-based pigment (a brightly colored transparent or semitransparent dye with a white pigment) is used as the colorant in most modern paints requiring either a highly saturated yellow or simplicity of color mixing. The most common is the monoazo arylide yellow family, first marketed as Hansa Yellow.

Dyes

Curcuma longa, also known as turmeric, is a plant grown in India and Southeast Asia which serves as a dye for clothing, especially monks' robes; as a spice for curry and other dishes; and as a popular medicine. It is also used as a food coloring for mustard and other products.

Saffron, like turmeric, is one of the rare dyes that is also a spice and food colorant. It is made from the dried red stigma of the crocus sativus flower. It must be picked by hand and it takes 150 flowers to obtain a single gram of stigma, so it is extremely expensive. It probably originated in the Mediterranean or Southwest Asia, and its use was detailed in a 7th-century BC Assyrian botanical reference compiled under Ashurbanipal. It was known in India at the time of the Buddha, and after his death his followers decreed that monks should wear robes the color of saffron. Saffron was used to dye the robes of the senior Buddhist monks, while ordinary monks wore robes dyed with Gamboge or Curcuma longa, also known as Turmeric.

The color of saffron comes from crocin, a red variety of carotenoid natural pigment. The color of the dyed fabric varies from deep red to orange to yellow, depending upon the type of saffron and the process. Most saffron today comes from Iran, but it is also grown commercially in Spain, Italy and Kashmir in India, and as a boutique crop in New Zealand, the United Kingdom, France, Switzerland and other countries. In the United States, it has been cultivated by the Pennsylvania Dutch community since the early 18th century. Because of the high price of saffron, other similar dyes and spices are often sold under the name saffron; for instance, what is called Indian saffron is often really turmeric.

Reseda luteola, also known as dyers weed, yellow weed or weld, has been used as a yellow dye from neolithic times. It grew wild along the roads and walls of Europe, and was introduced into North America, where it grows as a weed. It was used as both as a yellow dye, whose color was deep and lasting, and to dye fabric green, first by dyeing it blue with indigo, then dyeing it with reseda luteola to turn it a rich, solid and lasting green. It was the most common yellow dye in Europe from the Middle Ages until the 18th century, when it was replaced first by the bark of the quercitron tree from North America, then by synthetic dyes. It was also widely used in North Africa and in the Ottoman Empire.

Gamboge is a deep saffron to mustard yellow pigment and dye. In Asia, it is frequently used to dye Buddhist monks' robes. Gamboge is most often extracted by tapping resin from various species of evergreen trees of the family Guttiferae, which grow in Cambodia, Thailand, and elsewhere in Southeast Asia. "Kambuj" (Sanskrit: कंबुज) is the ancient Sanskrit name for Cambodia.

Food coloring

The most common yellow food coloring in use today is called Tartrazine. It is a synthetic lemon yellow azo dye. It is also known as E number E102, C.I. 19140, FD&C yellow 5, acid yellow 23, food yellow 4. It is the yellow most frequently used such processed food products as corn and potato chips, breakfast cereals such as corn flakes, candies, popcorn, mustard, jams and jellies, gelatin, soft drinks (notably Mountain Dew), energy and sports drinks, and pastries. It is also widely used in liquid and bar soap, shampoo, cosmetics and medicines. Sometimes it is mixed with blue dyes to color processed products green.

It is typically labelled on food packages as "color", "tartrazine", or "E102". In the United States, because of concerns about possible health problems related to intolerance to tartrazine, its presence must be declared on food and drug product labels.

Another popular synthetic yellow coloring is Sunset Yellow FCF (also known as 'orange yellow S, FD&C yellow 6 and C.I. 15985) It is manufactured from aromatic hydrocarbons from petroleum. When added to foods sold in Europe, it is denoted by E number E110.

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