The Fourth of July is this Saturday, and I can't resist trying to inject a little physics fun into the holiday celebration. My apologies to those of you who just wanted to sit in your backyard, eat a burger, and enjoy some fireworks without mentally calculating the trajectory of your bottle rocket or trying to guess at the chemical composition of your Roman candle.
Let's start with colors. Say you're a pyrotechnician. (Wow, how cool would that be? But seriously, don't try this at home. This is a thought experiment, merely to entertain you while you munch your veggie burger and sip your kool-aid.) How do you get the red, white and blue sparkles of a patriotic display? Well, it all comes down to having the right ingredients. When heated, different chemicals give off different colors. Copper ingredients give you a brilliant blue, and even a tiny amount of sodium will blaze with yellow. This is all thanks to things happening at a tiny scale, among the ingredients' atoms. One of an atom's electrons can absorb heat, which excites it to a higher energy state for a while. When it comes back down, it releases energy in the form of a photon—light! Whole molecules can also absorb heat, which causes the atoms to vibrate relative to each other and give off photons. Most colors besides yellow are due to some combination of the two.
But making a rocket requires more than just including the final ingredients you need. For instance, strontium chloride and strontium hydroxide make a wonderful red, but they're too volatile to start with. Instead, you need to plan the perfect reaction that's going to give you the maximum explosion, with the correct end products.
One important ingredient is the oxidizer (potassium perchlorate, for example, is nice and stable), which will produce a large amount oxygen in the reaction, and another is a reducer, which will burn the oxygen. (Pine root pitch is a popular choice). Strontium carbonate will result in getting the right color (due to strontium chloride) along with a lot of oxygen to burn:
Once you'vefound the alchemy that achieves a big explosion and "aaah"-inducing color, you would mix up the ingredients, with help from a binding agent, into a pellet called a "star." "Stars" can be as small as peas or as big as strawberries. Then you would fill your rocket with a black powder propellant, add your stars, and make sure the fuse is long enough to get your rocket into the air before the show begins.
Another pretty incredible thing that pyrotechnicians have to keep in mind for a big fireworks show is ballistics. How fast should they be traveling? How high will they go? How do you aim a hundred of them just right to get that shimmering bust of Uncle Sam? It's like your high school physics projectile motion lesson, but just a little bit cooler. One simple concept is that a bigger shell gives you faster take-off. This means the bigger rocket will fly to a greater height in the time it takes for the fuse to burn completely. Depending on how you pack your firework, you can get a range of effects: a halo, a palm tree, or even a weeping willow. Here's a really fun page that lets you see all the possible shapes. And if you want to impress your family and friends with your ability to determine the size of distant fireworks, check out this guide; all you'll need are some good fireworks to watch and your knuckles.
Have a wonderfully physics-packed Fourth of July!