You mean those plastic rods full of liquid chemicals that are made by Omniglow and other companies and are sold at street fairs, festivals and concerts and that start glowing with green, yellow or blue light when you bend them, and that gradually lose their light after an hour or so? Never heard of them.
By now you know that a fluorescent dye needs to be stimulated by absorbing energy before it can re-emit that energy as light. But the stimulating energy need not be visible light or ultraviolet radiation; it can also be heat, electrical or chemical energy.
In the case of the light sticks, the stimulating energy is chemical. When you bend the stick, you break a thin glass capsule containing a chemical, usually hydrogen peroxide, that reacts with another chemical in the tube. The reaction gives off energy, which is taken up by a fluorescent dye and re-emitted as light. As the chemical reaction gradually plays itself out because the chemicals are used up, the light fades.
At several places in this book I talk about a substance’s absorbing certain colors or wavelengths of light. You may be wondering how molecules actually absorb light, and what determines which wavelengths they absorb. If that problem is not exactly keeping you awake at night, Nitpicker’s Corners are designed to be skippable.
A molecule has custody of all the electrons that belong to the atoms that it is made of. (Molecules are nothing but atoms glued together.) But electrons, and for that matter all subatomic particles, have a peculiar property: They can have only certain amounts of energy and no others. (Techspeak: The electrons’ energies are quantized.)
For example, the electrons in a certain kind of molecule can have energies A, B, C or D, etc., but never A-and-a-half or C-and-two-thirds. They can change their energies up or down among the values A, B, C, D, that is, from A to B or from D to C and so on, but they can never have values anywhere in between. Nobody can give you a reason for this; that’s just the way it is. When you get down to things smaller than an atom, it’s a different world from the one we see every day up here in big-land.
Now inasmuch as each unique substance is made up of its own unique molecules, it will have its own unique collection of electrons with their own unique sets of allowable energies.
When light energy falls upon the substance, its electrons will absorb only those energies that correspond to its allowed energy jumps from A up to B or C, etc; it will reject and reflect the rest. This means that the substance is actually picking out the light energies (wavelengths) that it prefers, leaving the others to bounce back as reflected light.
And that’s why every colored substance has its own color: the color of those wavelengths that it cannot absorb and that it reflects back for our eyes to see.