How Fluorescence Works - The Science 

In this video we explore the colorful science of fluorescence.
A really cool way to play with fluorescence at home is get a blue or violet laser pointer and shine it into a dish or jar of water where you have added a drop of fluorescent highlighter fluid. You'll clearly see the beam as the solution fluoresces in its path.
Now the common definition of fluorescent is something that glows a visible color when exposed to ultraviolet light. Fluorescence is actually much broader than that and you don't need ultraviolet light in particular. For example in the yellow fluorescent dye "rubrene" both violet and green lasers will activate it and glow yellow. This proves you don't always need ultraviolet light. But a red laser will not activate a yellow dye. Why is that?
What's happening in fluorescence is that the incoming light raises the energy of the electrons in the molecule to an excited state. The electrons then lose a bit of energy due to vibrations of the molecules. And finally the electrons return to the ground state by releasing light. Now since energy cannot be created or destroyed and a bit of energy was already lost as heat in the vibrations of the molecules, the energy of light emitted must have lower energy than the light absorbed.
So since the yellow fluorescent dye emits yellow light, we need to use light of higher energy like violet and green for it to glow. Red is lower energy than yellow light so it can't excite the dye.
It also won't work if you use the same color as the dye like a green laser onto a green dye. This is because you almost always lose a bit of energy and therefore it has to emit a different color or none at all.
Another restriction is that the dye has to also absorb the light to work.
For example Europium Tetrakis (Dibenzoylmethide)Triethylammonium will glow bright orange under violet light. But under green light it doesn't glow at all. This is because the substance simply doesn't absorb green light and so it can't reach an excited state where it can fluoresce. So you need both absorption and higher energy to get fluorescence. Higher energy does not automatically imply absorption.
A really cool trick with absorbance and fluorescence is to get multiple fluorescent dyes of differing colors and shining various wavelengths of light on them. If you start with short, high energy, wavelengths of light you will see all the dyes glow. But as you go to longer wavelengths of lower energy the high-energy dyes will go clear as they can no longer absorb or emit light. When you reach red light all the dyes may look "clear" even though you know they are actually multiple vivid colors in white light.
Now what happens if you mix fluorescent dyes? The results depend on the concentration, absorption and emission profile of the dyes. Sometimes the colors of the resulting fluorescence will simply add up and you get a mix of color. Other times the emission of the higher energy dye will get absorbed by the lower energy dye and the color you get at the end is exclusively the lower energy dye.
Moving on to a new topic now: I said fluorescence worked by first using light to push electrons into high-energy states. Well if we could just push these electrons into their higher energy levels then we don't necessarily need the light. We can do this in a glow stick. A glow stick reaction uses chemicals instead of light to excite the electrons in a fluorescent dye.
Now instead of using chemicals to excite those electrons we can also use direct mechanical grinding. A special range of compounds has this property called triboluminescence.
On a different note: Remember when i said way back that after an electron is excited it loses a bit of energy due to molecular vibrations? It follows that if we can alter or stop these vibrations then we can change the energy of fluorescence and thus its color.
A special dye called pyridine copper iodide exhibits a property called fluorescence thermochromism. It changes fluorescent color with temperature. At room temperature the color is yellow but under liquid nitrogen it changes to blue.
Finally a life saving application of fluorescence is in medical diagnostics. A dye is specially engineered to glow when it comes into contact with a pathogen. Since pathogens usually only occur in tiny invisible concentrations by using one of these special fluorescence dyes a sample to be tested quickly and accurately for the presence of such pathogens. The key point is that fluorescence can still be very bright even at these tiny concentrations.
So there you have it, a cool effect that helps us to understand light, have fun and save lives.