“Celebrate the independence of your nation by blowing up a small part of it.” -The Simpsons
For those of you who aren’t from the U.S. or U.K., this coming Monday is the day that my nation celebrates the birth of its independence. This date, of course, as Aimee Mann will sing to you, is the
You start with three simple ingredients: sulfur, charcoal, and a source of potassium nitrate. Charcoal, in this case, is not the briquettes you use on your grill, which often contain no actual charcoal, but is the carbon residue left behind by organic matter (like wood) once it has been charred (or pyrolyzed), and all the water removed. Potassium nitrate is found in sources like bird droppings or bat guano. Take a mortar and pestle, mix them together, and what you’ll get is a fine, black powder.
Gunpowder, in fact. All you need now is some oxygen — readily found in our atmosphere the potassium nitrate source (see here, and thanks derek @1) — and a small source of heat. (A match will do.) Put it all together?
Well, you’ll get an explosion (and a loud boom), but that’s hardly a firework! After all, if you’ve ever seen one, you know that the four major things that make a good firework are height, size, shape and color.
Physics has something to say about each one of these! The height is the easiest one, so let’s start there.
The way you launch a firework is basically the same way you launch a cannonball out of a cannon! You put a “lift charge” in between the actual firework and the bottom of a strong, closed tube/pipe, and ignite it, propelling the firework up.
How high you want it to go is dependent only on the initial velocity of your firework, which is almost always larger for bigger fireworks.
A small fireworks show might have 2″ (5 cm) to 6″ (15 cm) diameter shells being launched, which reach a height of anywhere from 200 to maybe 500 feet (60-150 m). But a very large fireworks show, like the one in New York City, uses fireworks with shells up to two or three feet in diameter (up to nearly a meter), and those fireworks often reach altitudes of well over 1,000 feet (300 meters).
Once the initial launch happens, the fuse on the actual firework itself — if all goes properly — is now lit, and burns as it goes up.
For aesthetic and safety reasons, you launch the larger fireworks to a higher altitude. The physics helps out tremendously with the size of your fireworks as well, because a larger firework requires a larger lift charge! And the amount of the lift charge that you use is sufficient to launch it to the necessary altitudes described above, which is why larger fireworks get launched to higher altitudes.
So long as they’re not “duds” (i.e., so long as the fuse ignites and burns properly), they will explode at or near the apex of their flight. The higher ones are usually larger, resulting in, well, aesthetically-pleasing (and again, safer) fireworks displays. But what determines their spectacular shapes that they come in? To find that out, we need to go inside the anatomy of a firework.
Fireworks come in many different styles, but the two important elements, once your firework has been launched into the air with its fuse lit, are the burst charge and the stars.
The burst charge can be as simple as more gunpowder, or it could be a more complicated (or even a multi-stage) explosive. The stars, on the other hand, are what actually go off in many directions, producing the beautiful display. When the fuse burns down to the point where it reaches the burst charge, it ignites!
This ignition, depending on how the firework is put together in the first place, will send the stars off into whatever pattern or direction it was designed for.
It will also reach high enough temperatures to ignite the individual stars. This is where — for me — the most interesting part of the fireworks happens. In addition to whatever (optional) propulsion or fuel exists inside these stars, such as the ability to make them spin, rise, or thrust in a random direction, the stars are also the source of the light and color we find in our fireworks.
How are these “stars” responsible for color? Although there are some recent advances (covered in excellent detail by Janet), the simplest explanation is that different elements and compounds have different characteristic emission lines. For example, if you take some sodium and heat it up, it emits a characteristic yellow glow, because of its two very narrow emission lines at 588 and 589 nanometers. (You’re probably familiar with them from sodium street lamps.)
Well, we have a great variety of elements and compounds that emit a great variety of colors! Different compounds of Barium, Sodium, Copper and Strontium can produce colors covering a huge range of the visible spectrum, as shown below (thanks to this site) in chromaticity space.
And that’s how fireworks work, from launch, up to the proper height, to their explosion, to the size, pattern, and color of the spectacular show they put on!
What more can you ask for? Alright, how about the boom sound that accompanies it? Believe it or not, it’s the same principle that the sound of lightning — thunder — operates on! Update: No, it doesn’t!
Just like lightning, the gunpowder explosion superheats the air, causing it to expand and become very rarefied (i.e., have such a low density that it’s almost a vacuum). This heating and expansion takes place in a tiny fraction of a second, but it’s not the heating and expansion that makes the sound! It’s what happens in the instant afterwards. Although this is a reasonable explanation for how lightning makes a sound, fireworks don’t go through all of this. It’s actually much simpler.
The air from just outside this rarefied area rushes in to fill up this low pressure area. It does so with such great speed that it actually breaks the speed of sound, creating a sonic boom! And that’s what makes the “pop” or “boom” sounds that accompanies fireworks. The “blast” from the explosion of gunpowder creates an outward moving pressure wave. At the front of this wave, this pressure reaches very high levels, many times that of the normal atmosphere. The “boom” you hear is simply the rapid change in pressure in the air, which is all that sound — a pressure wave — really is. (Thanks to daedalus2u in the comments for catching this egregious mistake!)
And that’s the physics of fireworks! Now, go enjoy the show!