Crystals - 2. For example, if a molecule absorbs mostly blue light, then it will look yellow because there is more yellow light reflected after the blue has been absorbed. But visible light spans only a tiny fraction of the full range of possible wavelengths.

Crystals- Ever wondered why your jeans fade but the pigment in cave paintings lasts for thousands of years? Only a few natural substances have the right kind of chemical structure to absorb visible light, like these crystals of arsenic sulphide realgar

Only a few natural substances have the right kind of chemical structure to absorb visible light, like these crystals of arsenic (realgar) White light is a mixture of all colours, and when we say a compound is coloured, we mean that it reflects light of one colour (which corresponds to light of a particular range of wavelengths) more strongly than light of other colours.

Crystals- Ever wondered why your jeans fade but the pigment in cave paintings lasts for thousands of years? Only a few natural substances have the right kind of chemical structure to absorb visible light, like these crystals of arsenic sulphide realgar

cADMIUM YELLOW will keep Van Gogh's sunflowers gold forever. Image Wikipedia

Cadmium yellow will keep Van Gogh's sunflowers gold forever. (Image Wikipedia) Ever wondered why your jeans fade but the pigment in cave paintings lasts for thousands of years?

Van Gogh Another name for brightly coloured stones is 'gems', or at least 'semiprecious stones'. That sort of pigment, ultramarine derived from the gem lapis lazuli for example, can be very expensive, which explains why some colours were available only to the wealthy until the invention of a full spectrum of synthetic organic dyes.  Perhaps Van Gogh painted a lot of sunflowers not because he liked them but because cadmium sulphide (cadmium yellow) was available. It's highly toxic, but … art.

Cadmium yellow will keep Van Gogh's sunflowers gold forever. (Image Wikipedia) Ever wondered why your jeans fade but the pigment in cave paintings lasts for thousands of years?

Tartrazine A persistent problem with many organic dyes like indigo and tartrazine is that they are not lightfast. They fade over time because the fact that they absorb the energy of visible light — rather than reflect, deflect, or entirely ignore it - means that they are vulnerable to damage by that very light. Their colour originates from delicate chemical structures and goes away if those structures are broken.

Tartrazine is one of the synthetic food colorants responsible for the garish hues of lollies. The nitrogen double bond in its middle (N=N) gives rise to its bright orange colour.

Light spectrum We only see molecules that absorb light in the visible range. Most organic compounds absorb ultraviolet (UV) light, so they appear white to us, but look colorful to creatures like bees that can detect UV.

Colour is all around us, but almost all pure compounds are white.


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