Science with Richard Bleil
Today had a nice little surprise. I’ve started working at a Drive-In Theatre recently. Towards that end, they gave me a t-shirt with the name and logo on it of the Drive-In on it. At the end of my shift, I clean the men’s room. Having done so, I was heading back to my car, of course in the dark, when I noticed that the ink on the t-shirt glowed.
Glowing is a fascinating process. It’s related to fluorescence, with which you are probably familiar. When you’ve worn something and noticed it seeming to glow in a black light, that’s fluorescence. The difference between glowing and fluorescence is that glowing lasts longer, on the order of minutes or even hours.
Don’t confuse fluorescence and glowing with chemiluminescence, although, it, too, is related. In chemiluminescence (also called bioluminescence if it happens in a biological organism such as a firefly or jelly-fish), a chemical reaction is what gives rise to the glow (more specifically, a chemical oxidation reaction). But fluorescence is different.
You may have heard of quantum mechanics determining electronic structure. When we talk about electronic structure, we are usually talking about the “ground state” structure of electrons. Ground state energy is exactly as it sounds, the ground state. This is the lowest possible energy of the system. If you are standing on the ground, you have no potential energy. You cannot get to a lower level, or lower potential energy. It’s the very ground. We also talk about excited states.
Excited states are always higher potential energy than ground state. If you were standing on a table, you could fall. Your potential energy is higher than the ground state. Although there is one ground state, there are multiple excited states. You could stand on a table, or stand on a chair, or stand on a ladder, all of which have different potential energies because they are all different heights. As you jump off of one of these heights, you can land on something that is higher than the ground, but lower than the object from which you jumped. This is still an excited state.
Energies in atoms and molecules have different energy levels. Just like the potential, there’s only one ground state (lowest energy), but multiple excited states (always higher energy than the ground state). Now, if you wanted to climb on top of a table (or chair or ladder), you have to expend energy. It takes effort to get to a higher potential energy. However, the exact same amount of energy is released again in the fall when you jump back down. The height between the table and the ground is fixed, so the same energy to climb up is what is given off when you jump off, only in opposite directions. You have to expend energy to climb, but the energy when you leap off is felt in the impact with the ground.
Electrons are exactly the same. They must absorb energy to go from the ground state to an excited state. This energy is in the form of light energy. Different frequencies of light (colors) have different energy, which is why my t-shirt is orange. It’s absorbing light energy in the red, yellow, green, blue, indigo and violet frequencies, while reflecting energy in the orange frequency. This makes the shirt orange.
When the electrons “relax” back to a lower energy (such as the ground state), light is then released. But, just like different objects of various heights that you can climb on, the electron doesn’t have to drop back down to the ground level. They can fall to another, but lower energy, excited state energy.
Now, when the electron falls to another energy level, a different energy is released than the energy that caused the electron to go to the original excited state. Because the energy is different, the frequency of light that is released, or color, is also different. This is why my shirt might absorb, say, blue light, but glow green. It’s simply glowing at a different state than the ground state.
The difference between fluorescence and glowing in the dark is time. Fluorescence happens almost instantaneously, while glowing takes much longer. It’s like an atomic battery. Energy is absorbed in the light, and energy is released in the dark. The energy is actually released continuously, even in the light, but we don’t notice it because it’s, well, light. It’s all a beautiful ballet of electrons dancing on various energy levels in the subatomic scale.
Bleil's Banter 