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How Does Glow-in-the-Dark Stuff Work?

If you’re a member of my generation or the one that raised it, your house was probably full of all sorts of glow-in-the-dark stuff in the 80s and 90s. Yo-yo's, stickers, action figures, clothing, you name it. As a kid, I thought it was just short of magic. The effect is less impressive to adult me, but the chemistry behind it is pretty cool.

Your average glow-in-the-dark doodad gets its glow from a phosphor, a member of a group of substances that radiate visible light after being energized. Some phosphors are natural, like ones found in your teeth and fingernails, and chemists have also created hundreds of others. The ones most useful for glow-in-the-dark items are those that can be energized by normal light and have a pretty long persistence (glow time).

Take a phosphor that fits the bill, mix it in with the plastic to be molded into the product, and you have yourself a glow-in-the-dark whatever. Light from the sun or the living room lamp energizes the phosphors in the plastic and excites them, and with the lights off, you can watch as their atoms slowly lose this extra energy in the form of a dim glow.

Beyond the usual glow-in-the-dark artifact, there are some special cases where glowing products work a little differently. Glow sticks work by chemiluminescence — that is, the light is emitted as a product of a chemical reaction. Items that need to glow continuously with little or no “charge,” like clock or watch hands that glow for hours after a light has been turned off, work by radioluminescence. Timepieces like this still use phosphors to create the glow, but also have a little bit of a radioactive element like radium added to the glowing parts, which gives off small amounts of energy — not enough to be dangerous to the user, but, historically, a problem for the people who make the products — that constantly charge the phosphors in the same way a light would and keep the item glowing through the night.

* Phosphors played another large role in my childhood besides powering my glowing toys, and helped create the images on the displays of my grade school’s ancient computers. In other words, you can thank them for your first Oregon Trail experience.

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Big Questions
Why Is Holly a Symbol of Christmas?
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Santa Claus. A big ol’ red-and-white stocking hung by the fire. Nativity scenes. Most classic Christmas imagery is pretty self-explanatory. Then there’s the holly, genus Ilex, which found its way onto holiday cards through a more circuitous route. 

Christmas is kind of the new kid on the block as far as holly symbolism is concerned. The hardy plant’s ability to stay vibrant through the winter made it a natural choice for pre-Christian winter festivals. The Roman feast of Saturnalia, celebrated at the darkest time of the year, celebrated the god of agriculture, creation, and time, and the transition into sunshine and spring. Roman citizens festooned their houses with garlands of evergreens and tied cheery holly clippings to the gifts they exchanged.

The Celtic peoples of ancient Gaul saw great magic in the holly’s bright "berries" (technically drupes) and shiny leaves. They wore holly wreaths and sprigs to many sacred rites and festivals and viewed it as a form of protection from evil spirits. 

Christianity’s spread through what is now Europe was slow and complicated. It was hardly a one-shot, all-or-nothing takeover; few people are eager to give up their way of life. Instead, missionaries in many areas had more luck blending their messages with existing local traditions and beliefs. Holly and decorated trees were used symbolically by new Christians, just as they’d been used in their pagan days.

Today, some people associate the holly bush not with the story of Jesus’s birth but with his death, comparing the plant’s prickly leaves to a crown of thorns and the berries to drops of blood. 

But most people just enjoy it because it’s cheerful, picturesque, and riotously alive at a time when the rest of the world seems to be still and asleep.

NOTE: Holly is as poisonous as it is pretty. Please keep it away from your kids and pets.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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What Are the 12 Days of Christmas?
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Everyone knows to expect a partridge in a pear tree from your true love on the first day of Christmas ... But when is the first day of Christmas?

You'd think that the 12 days of Christmas would lead up to the big day—that's how countdowns work, as any year-end list would illustrate—but in Western Christianity, "Christmas" actually begins on December 25th and ends on January 5th. According to liturgy, the 12 days signify the time in between the birth of Christ and the night before Epiphany, which is the day the Magi visited bearing gifts. This is also called "Twelfth Night." (Epiphany is marked in most Western Christian traditions as happening on January 6th, and in some countries, the 12 days begin on December 26th.)

As for the ubiquitous song, it is said to be French in origin and was first printed in England in 1780. Rumors spread that it was a coded guide for Catholics who had to study their faith in secret in 16th-century England when Catholicism was against the law. According to the Christian Resource Institute, the legend is that "The 'true love' mentioned in the song is not an earthly suitor, but refers to God Himself. The 'me' who receives the presents refers to every baptized person who is part of the Christian Faith. Each of the 'days' represents some aspect of the Christian Faith that was important for children to learn."

In debunking that story, Snopes excerpted a 1998 email that lists what each object in the song supposedly symbolizes:

2 Turtle Doves = the Old and New Testaments
3 French Hens = Faith, Hope and Charity, the Theological Virtues
4 Calling Birds = the Four Gospels and/or the Four Evangelists
5 Golden Rings = the first Five Books of the Old Testament, the "Pentateuch", which gives the history of man's fall from grace.
6 Geese A-laying = the six days of creation
7 Swans A-swimming = the seven gifts of the Holy Spirit, the seven sacraments
8 Maids A-milking = the eight beatitudes
9 Ladies Dancing = the nine Fruits of the Holy Spirit
10 Lords A-leaping = the ten commandments
11 Pipers Piping = the eleven faithful apostles
12 Drummers Drumming = the twelve points of doctrine in the Apostle's Creed

There is pretty much no historical evidence pointing to the song's secret history, although the arguments for the legend are compelling. In all likelihood, the song's "code" was invented retroactively.

Hidden meaning or not, one thing is definitely certain: You have "The Twelve Days of Christmas" stuck in your head right now.

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