- Material, meaning things made up of a wide range of molecular components in any of the four major states (solid; liquid; gas; plasma);
- Responding to, meaning displaying a quantifiable reaction to in some manner.
- Specific light, meaning light which has a specific wavelength; or, more scientifically speaking, a specific energy level.
Putting that all together then, fluorescence is a wide range of things reacting to light of a specific wavelength by emission of light, in this case also of a specific wavelength.
The most commonly observed phenomenon that we associate with it is the response of white fabrics to blacklight in a disco; that eerie purple/blue glow is the result of near-UV (NUV) fluorescence, where the white fabric absorbs some of the blacklight’s energy and then re-emits its own light with marginally less energy. The net result is that the wavelength is increased – that is, the wave’s interval increases to some specific, longer one, like from green to red or violet to cyan.
It doesn’t need to be a huge shift, though. The difference (the Stokes shift) can be so small our eyes can’t really distinguish it – as small as a handful of Angstrom, a measure of wavelength even more granular than the more common nanometre (nm) scale. On the other end, there’s super- and metamaterials which exhibit a shift on the neighbourhood of 500-600 nm – in the ballpark of 1000 times broader shift.
Fluorescence is most commonly associated with UV-induced Visible Fluorescence (UVIVF), where the activating light – the excitation – is in the UV range, and the outputted light – the emission – is in the visible light range. Other kinds exist, with a significant portion of marine fluorescence being excited by NUV and blue light.
We can use this process to make colours seem brighter than is physically possible, like white paper that has increased contrast because it has brightening agents like Titanium dioxide (TiO2) added. A large number of plastics, toothpastes, and even foods have similar agents in them to make them brighter. Once you know what it is, you start to notice it everywhere; it’s no stretch to say that modern life in many ways wouldn’t be possible without exploiting it.
Nature, it seems, had very much the same idea. Biofluorescence is exactly what it sounds like, bio– (living stuff) –fluorescence (fluorescence). As far as we can tell, it’s existed for about as long as life has been complex, likely evolving more than a billion years ago as photosynthesis progressed to being an oxygenic process. We’ve known about it in animals for a couple of hundred years, but we never really realised quite how much does it. Birds, bees, grass, trees, dogs, cats, your mother, your beaver skin hat. Genuinely, the universe has hidden some very magical things in that 99.99999% of reality we can’t perceive.
For most of what does it, we have only vague ideas on the mechanisms for how they’re doing it, and even less idea why. It’s an exciting frontier hanging at the intersection of quantum physics, biology, chemistry, and magic, and has very real conservation and monitoring implications.