SmartWater has a chemical fingerprint that links to a crime scene and is only visible under ultra violet light.

BBC: Prestbury war memorial

The action of ultra-violet light from the sun on a molecule called 7-dehydrocholesterol, found in the skin, turns it into the vitamin.

ECONOMIST: Schizophrenia: Let the sun shine in | The

One moment, the ethereal white or green curtains of light with a streak of violet take on forms that evoke the ancient mythology of the north - a palace of lights, a Sami fire in the wilderness, the prow of a Viking ship.

BBC: The perfect trip: Norway