What is light?

White light, such as sunlight, is really a mixture of all the colors of the rainbow. You can tell for example by letting sunlight pass through a prism. When the light beam passes through the interface between air and glass, it will bend. But the various colors of light will not bend equally, which causes the light to beam to divide and give rise to a rainbow pattern. The fact that we sometimes can see a rainbow in the sky is also due to bending of the light, in the interface between air and raindrops in the atmosphere.

Our vision depends on light, coming from for example the sun or a lamp, being reflected off objects around us and entering the eye. Which color we think an object has depends on what happens when light hits the object. If all the colors of the light are absorbed (caught) we think of the object as black, meaning that no light of the spectrum is reflected. If all the colors are reflected equally by the object, we see it as white. If the object absorbs red light and reflects the rest we see it as green, since green is the complementary color of red. If the object on the other hand absorbs green light, we will see it as red!

Opposite sides of the wheel are complementary colors, e.g. violet – yellow or red – green.

How do molecules interact with light?

All objects consist of molecules and some of these molecules are affected by visible light. These molecules will then determine how our eyes see the object. It is the structure of the molecule – how the atoms are connected to each other – that determines its ability to absorb light. Some configurations cause the molecule to absorb red light, others cause it to absorb blue light – or, more common, not to absorb light in any part of the visible spectrum.

Is there light that we cannot see?

Yes, there is! Sunlight, for example, contains ultraviolet radiation. This is light that our eyes cannot detect, but it affects our skin – which we notice when we get a tan! The eyes of animals are different from ours, and insects and some birds can for example see ultraviolet light. The world thus looks very different to them!

Man and dyes

Man’s fascination for colors is nothing new. Natural dyes can be found in plants as well as in animals and minerals. Long before there were chemists creating dyes in laboratories, there were dyers extracting colors from flowers, leafs, roots, the inner and outer bark of trees as well as their heartwood. The extraction and preparation of dyestuffs is mentioned in Chinese literature as long ago as 2600 B.C, and it may have been one of man’s earliest attempts at the practice of chemistry.

Compared with red or yellow colors, blue colors are very rarely found in plants and were valuable in the old days. Indigofera tinctoria, or True indigo, was known to be a major source of blue color.

In this video we tell the story of this color!

Indigofera tinctoria

Indigofera tinctoria can grow up to two meters and was first used in India to color textiles. The Indians exported the dye to the Greeks, who named it indikon, meaning “Indian substance”. The Romans used the term indicum, which passed eventually into English as the word indigo.

The leaves of I. tintoria are not blue but after fermentation under alkaline conditions followed by oxidation, the blue color appears. This process was discovered by numerous  cultures around the world, possibly when plant leaves were soaked in urine or covered with ashes, then left to ferment.

The molecule found in the plant is indican, which is colorless. During fermentation, the glucose unit is split off and the indoxyl molecule is produced. Indoxyl reacts with the oxygen in the air to produce blue-colored indigo.

The blue-colored indigo is, however, not soluble, and therefore does not attach to textiles. For the dye to be able to attach to textiles you have to reduce it to “white indigo” or leuco-indigo. Leuco-indigo is colorless, but attaches to textiles.

To dye a textile with indigo, you first reduce the indigo in a bath (typically with NaOH and a reducing agent), and soak the textile in it. It will become green-yellowish. When you pick the textile out of the bath and it gets in contact with oxygen, the colorless leuco-indigo is oxidized back to the blue indigo.

Synthetic indigo

Indigo was manufactured labour intensive methods for centuries. It was not until the end of the 19th century that a synthetic form of indigo became available. In 1865, the German chemist Johann von Baeyer began investigating the structure, and he found a way to make it in the laboratory from easily obtainable starting materials. By 1897 indigo became commercially available by the German chemical company Badishe Anilin und Soda Fabrik.

Today, the annual production of indigo is over 13,000 tons. The synthetic indigo, as well as the natural compound, lacks color-fastness. It is most often used to dye blue jeans where this property is considered a fashion advantage.