Paper...That Glows?!

On October 11, your favorite neighborhood team of scientists was out on Main Street in Mesa, AZ for the monthly Second Friday event. Every time we set up our booth we try to have a demonstration of some kind to entice people to come over and visit us. Personally I think science is a lot more fun when we can touch it (which is definitely part of why I'm a geologist), and it's good to have a focal point for conversation to put people at ease when talking to scientists. This month, one of our members, Don Balanzat, brought phosphorencent paper to our booth. What ensued was a lot of experimentation and qualitative quantum mechanics!

Phosphorescent paper displays a property called photoluminescence, which is a fancy way of saying "glowing." The "photo-" part of the word refers to photons, which are carriers of light. Most of us have heard of the electromagnetic spectrum, which is a way of classifying different types of radiation based on their wavelengths (the distance between two wavecrests) and their frequencies (the number of waves that pass a certain point, measured in Hertz/Hz). One fun fact is that wavelength is the inverse of frequency, which is a great thing to know if you like trivia.

The electromagnetic spectrum. From: GSFC/NASA.

In physics, light is considered both a particle and a wave. The particle is the photon, and the wave is how we model the photon's path and velocity. This is also why if you ever look at quantum mechanics equations, you'll see a lot of sine and cosine terms--turns out they are good base models for describing how light moves. Trigonometry is good for more than just triangles!

We did some qualitative (no equations, but still figuring out what's going on) experiments on the paper with some custom 3D printed Halloween cookie cutters that Don brought along with the paper. See the progression of 1) before we shined light onto it, 2) while we shined the blue flashlight onto the paper and cookie cutters, and 3) after we turned off the light and removed the cookie cutters.

Picture 1: Before we shined the blue light on the paper.
PC: Jessica Noviello.

Picture 2: While we shined the blue light on the paper.
PC: Jessica Noviello.

Picture 3: After we shined the blue light on the paper.
PC: Jessica Noviello.

Do you notice the shape of the cookie cutters is left behind? That's because the cookie cutters blocked the blue light from reaching the paper. The molecules underneath didn't absorb any photons, so their electrons didn't get excited. The phosphorescence eventually comes back to normal after ~20 seconds, but it's cool to see quantum mechanics at work!

What's going on here? As the photons hit the paper, the molecules that make up the paper absorb the light. All materials do this, by the way, even if it's rare to see something glow afterwards. The difference with the phosphorescent paper is, of course, the glowing after the light is removed. Different materials are more sensitive to different wavelengths of light. The paper we had was very sensitive to blue light, which is higher energy (shorter wavelength) light. It barely responded to the yellow-ish light of the nearby streetlamps and didn't notice red light (considered low energy due to its longer wavelengths).*

When the light source is removed, the molecules are slow to give up their photons. Even when the photons are released, they do not have the same energy or wavelength. This is due to something called energy states in atoms, the building blocks that make up molecules. An atom absorbing a photon is called "excited," and it shoots up an electron to a higher energy state. It's like someone perking up after having a cup of afternoon coffee. As that caffeine wears off, the energy level of the person goes back to its normal, lower, more stable state. This is what happens with the electrons when they give up their extra energy. In phosphorescent materials, it happens on a timescale we can observe.

Here are some more pictures from one of our experiments with another one of our props, a dinosaur.

PC: Jessica Noviello

PC: Jessica Noviello

PC: Jessica Noviello

*Author's Note: Even though I wrote "short" and "long," I am only talking about the light that we can see in the visible part of the spectrum. There's a lot more and, presumably, the paper would respond to things like UV and X-ray light, but we can't see that so we can't measure it without special tools, and we didn't have any of those with us. They are expensive!