UV and fluorescence

Have you ever looked at fish in a tropical aquarium? Some of them seem to have markings or patches of colour that ‘glow’ more vividly than what you would expect. Those markings are fluorescent. Special pigments on the surface of the fish are absorbing invisible UV radiation and using the energy to give out extra visible light. That extra light, which is in addition to the normal amount of reflected light, make the markings appear brighter and more vivid. Tropical aquariums usually have special fluorescent lights that give out a small amount of UVA light as well as visible light to simulate actual sunlight. Fluorescence only happens while UV radiation is present. Normal incandescent light bulbs (those that use hot filaments to produce light) do not emit UV radiation, and the fish would appear comparatively dull in colour.

A bright blue, yellow and black stripped tropical fish.

Fluorescent markings

Some tropical fish have fluorescent markings that make them shine vividly in sunlight or artificial light containing UV.

Rights: Denis Burchill

UV light and the fluorescence effect can be used in a number of science-related activities:

Some particular kinds of minerals in rocks fluoresce with particular colours. Geologists use UV to identify the minerals and the rocks they are in. (In fact, the word ‘fluorescence’ comes from fluorite – a substance that glows blue in UV light.)
In forensic science, fluorecein (a common fluorescent dye) can be mixed with other chemicals and sprayed on surfaces at crime scenes to reveal blood splatter when UVA light is shone on the surfaces. UV can also cause other body fluids, bone fragments and sometimes fingerprints to fluoresce.
UV fluorescence can be used as a security measure – some countries’ banknotes have a watermark that can only be revealed under UVA light, all major credit card companies print invisible fluorescent writing or images on their cards, and special events sometimes use invisible fluorescent ink stamped on people’s hands to allow them to gain free re-entry to the event.
Special pens containing a clear fluorescent dye can be purchased to write owners’ names or identification numbers onhidden parts of their valuables. These would not be visible to thieves but would appear under UVA light if the police recovered the valuables.
Most manufacturers of washing powders and liquids add a small amount of fluorescent dye to their products. This makes ‘whites appear whiter’ – in daylight, the white clothes shine more vividly white due to the additional fluorescence. If you then wear those white clothes to a party that has a black light (a popular party light that emits UVA), the clothes will shine bright blue.
Manufacturers of photocopy paper also add a small amount of clear fluorescent dye to make their paper appear whiter.