‘Tis the season for eerie lights. At Halloween you’ll see glow-in-the-dark face paint, creepy decorations shining ghostly green under black light, and glow sticks dangling from the necks of trick-or-treaters.
These lights are different from sunlight or ordinary light bulbs. They’re low-intensity and viewed best in the dark. They’re a single color, and they’re cool to the touch.
What are they?
These “glow” lights are all examples of fluorescence. Fluorescence is a kind of light produced by a fluorescent molecule (or fluorophore) after it is charged with energy. Typically, the energy comes from electromagnetic radiation (EMR)–either visible light or short wavelength, high energy forms like ultraviolet and X-rays.
When you bombard a fluorophore with electromagnetic radiation (such as by shining a light on it), the fluorescent molecule absorbs the energy but doesn’t keep it. Instead, the fluorophore sends energy back out as EMR of a longer wavelength. In other words, it emits light of a different color.
This creates cool visual effects if the “light” used to charge the fluorophore is invisible. Black lights such as you’ll find at a Halloween store are an excellent example. Black lights are peculiar light bulbs that emit EMR in ultraviolet wavelengths that are mostly outside the range that the human eye can detect. Even when a black light is burning at full intensity, all we can see is a faint purple glow. But the energy is there, and if it shines on, say, a fluorescent skeleton decoration, the skeleton lights up. Because we can’t see the brilliance of the black light, but we can see the re-emitted light coming from the skeleton, the whole thing seems like magic.
But what about glow-in-the-dark T-shirts or watch faces that shine in total darkness?
This is another kind of fluorescence that’s properly called phosphorescence. Phosphorescence is delayed or slow fluorescence. As with fluorescence, phosphorescent substances first have to be activated by exposure to electromagnetic radiation. But instead of immediately emitting energy, they release their light gradually over time.
If you’ve ever had a glow-in-the-dark item, you’ve probably experimented with these properties of phosphorescence yourself. To get your item to glow with the highest intensity, you first have to charge it by shining a really bright light on it. The longer you charge it, the more energy it stores, and the longer it will glow later.
A third common example of fluorescence is glow sticks. Glow sticks are a clever way of packaging a fluorophore with a built-in energy source that the user can activate when ready.
As you might guess, the energy comes from a chemical reaction. Inside every glow stick is a brittle, glass-like tube that keeps two chemicals apart. When you bend a glow stick, you break the tube and the chemicals mix. They react, and the reaction releases invisible energy. The energy charges the fluorophore, and the fluorescent molecules glow.
Glow stick light is brightest at the beginning. It fades as the chemicals are used up. You can regulate the reaction rate, and the lifespan of your glow stick, using temperature. Like most chemical reactions, this one is accelerated by heat and slowed by cold. You can’t turn off a glow stick, but if you want to save some of the light for the next day, put the stick in a freezer. The reaction will slow dramatically, conserving the chemicals for later. When the stick is warmed again, the reaction will resume and the stick will brighten.
On the other hand, if you want a glow stick to stay illuminated at about the same level for the longest possible time, rather than burning brightly at first and then dimming, refrigerate it before you turn it on. This will slow the initial reaction and even out the light intensity over time.
Note that the fluorophore in a glow stick is not consumed. A glow stick will fluoresce under black light before and after it’s been used.