The modern English word blue comes from Middle English bleu or blewe, from the Old French bleu, a word of Germanic origin related to Old Dutch, Old High German, Old Saxon blāo and Old Frisian blāw, blau. The clear sky and the deep sea appear blue because of an optical effect known as Rayleigh scattering. 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. An optical effect called Tyndall scattering, similar to Rayleigh scattering, explains blue eyes; there is no blue pigment in blue eyes. Distant objects appear more blue because of another optical effect called atmospheric perspective.
Blue has been used for art and decoration since ancient times. The semi-precious stone lapis lazuli, coming from mines in Afghanistan, was used in ancient Egypt for jewelry and ornament and later, in The Renaissance, to make the pigment ultramarine, the most expensive of all pigments. It is the most important color in Judaism. In the Middle Ages, cobalt blue was used to color the stained glass windows of cathedrals. Beginning in the 9th century, Chinese artists used cobalt to make fine blue and white porcelain. Blue dyes for clothing were made from woad in Europe and indigo in Asia and Africa. In 1828 a synthetic ultramarine pigment was developed, and synthetic blue dyes and pigments gradually replaced mineral pigments and vegetable dyes. Pierre-Auguste Renoir, Vincent van Gogh and other late 19th century painters used ultramarine and cobalt blue not just to depict nature, but to create moods and emotions. In the late 18th century and 19th century, blue became a popular color for military uniforms and police uniforms. In the 20th century, because blue was commonly associated with harmony, it was chosen as the color of the flags of the United Nations and the European Union. Toward the end of the 20th century, dark blue replaced dark grey as the most common color for business suits; surveys showed that blue was the color most associated with the masculine, just ahead of black, and was also the color most associated with intelligence, knowledge, calm and concentration.
Surveys in the US and Europe show that blue is the color 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 color, chosen by almost half of both men and women as their favorite color.