In a Day-Glo colored object there’s a chemical that takes invisible ultraviolet radiation out of the daylight and converts it into visible light of the same color as the object.
Thus, the object is not only reflecting its normal amount of colored light, but is actively emitting some light of the same color, which makes it look “extra-colored” and up to four times brighter.
The DayGlo Color Corporation of Cleveland is only one manufacturer of what are called daylight fluorescent pigments. As the self-proclaimed world’s largest supplier, it makes a dozen different colors, from aurora pink to saturn yellow. It sells the pigments to companies that put them and similar dyes into everything from orange safety vests and traffic cones to yellow tennis and golf balls and highlighting pens.
What’s going on is fluorescence, a natural process by which certain kinds of molecules absorb radiation of one energy and re-emit it as radiation of a lower energy. The molecules in the pigment are absorbing ultraviolet radiation, a kind of short-wavelength radiation that human eyes can’t see, and re-emitting it as a longer-wavelength light that human eyes can see. The radiation is, in effect, shifted from invisible to visible.
How do molecules absorb and re-emit radiation? Molecules contain lots of electrons that have certain specific amounts of energy characteristic of the particular molecule. But these electrons are always willing to take on certain amounts of extra energy from outside. (For more on this point, meet me in the Nitpicker’s Corner.)
A molecule of a typical pigment may contain hundreds of swirling electrons of various energies. When a bullet of ultraviolet radiation (Techspeak: a photon;) hits such a molecule, it may kick some of those electrons up to higher energies. (Techspeak: The electrons become excited; honest, that’s what scientists say.) But they can hold on to their overabundance of energy for only a few billionths of a second (a few nanoseconds) before spitting it back out as radiation again, usually as several photons of lower energies or longer wavelengths. It’s sort of like spitting out buckshot after stopping a bullet.
Now the “buckshot” radiation, having somewhat less energy than ultraviolet radiation, falls into the region of radiations that human eyes can see: colored light. The net result is that the pigment molecule has absorbed invisible radiation and re-emitted it as visible radiation.
As long as the pigment molecules are being exposed to ultraviolet radiation, and daylight contains lots of it, they will be absorbing it and emitting light of a visible color. If the pigment happens to be orange to begin with and the emitted light is also orange, the dyed object will be an unnatural super-orange, “oranger” than you think it has any right to be.
Shine an ultraviolet lamp, a so-called black light lamp, on a Day-Glo fluorescent object, such as a paper with a few streaks of fluorescent highlighter on it or, if you’re the type who wears them, your Day-Glo-printed T-shirt. The fluorescent dye will glow very much brighter than it does in daylight because the lamp puts out much more ultraviolet radiation. If you don’t want to buy an ultraviolet lightbulb, take your streaked paper or T-shirt into one of those tacky stores that sell tasteless gifts and fluorescent posters, and use their black light for free.
By the way, if you use a fluorescent yellow highlighter on your books or notes, remember that it is brightest in daylight, which contains plenty of ultraviolet.
Ordinary household incandescent lightbulbs give off very little ultraviolet light; moreover, their light is somewhat yellowish, and that washes out the yellow highlighter color. So when reviewing your highlighted book passages or notes by lamplight the night before an exam or a presentation, you may find to your chagrin that your highlighting is all but invisible. It’s safer to use the stronger highlighter colors: orange, green or blue, whether fluorescent or not.
In my work, just about the only excuse I haven’t heard from a student who did poorly on an exam is that the highlighting on his notes disappeared.
