One of the most popular classes at AstroCamp is Lights and Lasers, where students learn about the different energies and properties of light. The Lights and Lasers room is easily recognizable because of its Glow-in-the-Dark Walls. Once you turn the lights off, these awesome walls glow a vibrant green, slowly dimming until you shine light on them again.
These walls have a physical property known as phosphorescence. It is a type of photoluminescence: an emission of light occurring when an object absorbs and releases photons. Other common types include bioluminescence — a chemical reaction in living organisms that give off light — and electroluminescence — the process by which LEDs give off light.
Phosphorescence works by absorbing photons into the object’s electrons. This bumps those electrons into a higher energy level. But the electrons cannot then easily reemit the photons to return to their ground state: the electron becomes “trapped” and it requires a “forbidden transition” to return to its lower energy.
However, due to quantum mechanics, this forbidden transition can still happen, but it does so at a fairly slow rate. This allows the material to “store” the light and let it out slowly, sometimes taking hours to let out all of its light! This same material that we use for our glow wall is what is used for things like glow in the dark stickers! So even though they seem simple, next time you see them you’ll know that there’s a lot more going on than what first appears!
Light travels at different speeds through different media. Water slows light down more than air, for instance. Corn syrup slows it down even more than water. In this experiment, we create a gradient solution of sugar water in a tank. As a laser beam travels through the liquid, the changing concentration bends its path!
The solution at the bottom of the tank is about 80 percent sugar by volume. As a siphon slowly transfers liquid into the aquarium, we add more and more water to the top container, diluting the mixture. The last layers to be added contain almost no sugar.
Sugar water is denser than pure water. This property keeps the heavier, more concentrated solution on the bottom of the tank. Lighter mixtures float above in order of density. We chose a siphon system to stack our liquids because it transfers the fluid slowly and gently, without creating too much turbulence. This method allows a relatively smooth density gradient to form.
Our variable solution of sugar water also has a gradient index of refraction, meaning that light travels faster in some parts than others. Namely, light is slowed down the most in the heavy 80% region towards the bottom. It travels faster as the mixture becomes more dilute near the top of the volume of fluid.
Imagine a line of children holding hands and running across a field. If they all run at the same speed, the line stays straight. What happens if they’re not all identical athletes, and you place them in order of running speed? The line bends. One end of the chain ends up ahead of the other. This is a bit like what we’ve done to the laser beam.
Lasers are awesome! We use them for medicine, science, and even entertainment. But one of the most practical uses for a laser is the transmission of information. The information can come in a variety of different forms, from music to television to internet. This idea of information transfer just using light shouldn’t come as much of a shock. Radio stations have been transmitting information using light waves for over a century. Lasers just take this technology to the next level, allowing more data to be transferred at a quicker rate. While radio waves can pass through most everyday objects, laser light would be blocked and the information lost. To solve this problem engineers have invented the fiber optic cable.
A fiber optic cable is constructed in such a way that laser light cannot escape it, even though the cable is transparent. We call this “total internal reflection”, meaning any light from inside the cable gets bounced back into the cable. Long strings of fiber optic cables allow laser information to travel very far with very little loss of information. If you have HD television or high speed internet, chances are that you are using a fiber optic cable and you don’t even know it. Yay technology!
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