The Physics of Fireworks
Every firework is two physics acts: first a precisely aimed projectile launch, then a symmetrical explosion whose colors are written in atomic emission spectra.
The shell is a projectile
A mortar launches the shell like a cannonball; a time-delay fuse is set to ignite the burst charge near apogee, where velocity is lowest and the explosion looks most symmetric. Range and height follow the same R = v₀²sin(2θ)/g as any projectile.
Colors are atomic fingerprints
Heated metal salts emit light at characteristic wavelengths: strontium = red, barium = green, sodium = yellow, copper = blue. Each color is electrons dropping between specific energy levels — quantum mechanics as public art.
Why bursts are spheres
The burst charge sits at the shell's center, flinging 'stars' outward equally in all directions. In the shell's frame the explosion is symmetric, so you see a near-perfect sphere expanding.
Sound lags light
You see the burst instantly but hear it ~3 seconds per kilometer later (sound ≈ 343 m/s). Distant finales arrive as a rolling rumble as bursts at different distances report in.
Try this yourself
- Time the flash-to-bang delay and divide by 3 for the distance in km.
- Note which colors appear — identify the likely metal salt behind each.