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Why are things coloured?

What is colour?

We know that white is the result of all colours mixed together. On the other hand, black is the total absence of light sensation from a particular region. A black light ray cannot exist, because black is the absence of light.

One possible definition of colour is:

It is the result of a non-white distribution of light. We know that white light consists of all different colours mixed in approximately equal amounts. So, for a colour (other than white or black) to be observed, we need a substance (pigment) that selectively absorbs certain colours while reflecting the others. This way the white colour is destroyed , because not all the colours are present anymore, as some have been absorbed by the pigment. For this reason, a pigment is a very special molecule; we are used to see everything coloured nowadays, but this is a relatively new thing in human history. Ancient man would only have the colours of nature like in the green plants, in the blue sky and in the red soil.

The visible spectrum

Example: if we expose to white light a green pigment, like chlorophyll, the green colour is reflected and the other colours are absorbed. That is why we see it green! That means reflecting the colours in the middle of the spectrum and absorbing the ones on the edges, like red and purple. That also means that if we shine only green light on a typical plant, no photosynthesis will occur as a result because the chlorophyll (pigment that traps light for photosynthesis) reflects green light.

Chlorophyll (the yellow atom
is magnesium)

Why are plant leaves not black? If plants are supposed to absorb the maximum possible amount of light to favour photosynthesis, they should be black so that they could avoid wasting the green light that is reflected out. They are green because of evolutionary issues, as they had to compete with other life forms that were using the green light only, so they had to rely on the left over colours (the edges of the spectrum). To overcome this limitation, some plants developed other pigments like the carotenoids, which provide to leaves the various tones observed in autumn. Carotenoids are also present in humans; it participates in skin colour.Imagine a world with black plants and leaves... Maybe it is more beautiful as it is, with the various tones of green and also other colours.

How pigments absorb only particular colours? The various physical and chemical mechanisms that enable pigments and other materials to selectively absorb light are described here: (coming soon)

The history of man made colour

One of the first pigments used for manufacturing coloured things was called tyrian purple (image below). It was extracted from a snail and it could be used as a fabric dye. It was difficult to produce (10,000 snails gave 1 g of dye) and as a result its price was very high; that is why the purple colour was associated to powerful people. They were the only that could afford it and everybody else would dress colourless clothing.

ATP structure
Tyrian purple (colour code of atoms on
the left-Bromine is in brown)

Pigments were also produced by grinding colourful stones, like lapis lazuli (blue), an expensive stone. This was the only blue pigment that could be used by painters from around the 15th century; it was very expensive so that very few painters could draw a blue sky at the time. Furthermore, not even the rich painters could have a large variety of colours available to paint.

Pigments became cheap and widespread around the end of the 19th century, after the discovery of azo compounds (azo dyes), obtained from coal tar, that could be made in a wide variety of colours.