Every colour in a firework comes from a different metal hidden inside it. When the metal gets blisteringly hot, its tiny electrons jump up to a higher level — and the instant they fall back down, they fling out a flash of light. The clever part is that each metal can only ever throw out its own particular colours, like a fingerprint nobody else shares. Sodium, the metal in table salt, glows yellow. Copper glows a beautiful blue-green. Strontium burns a fierce red. Pick a metal in the simulator and watch its flame colour, along with the exact bright lines of light it gives off.
Most people think firework colours are coloured powders or dyes that burn. In fact the colour is a heated metal glowing, and each metal can only ever emit its own fixed set of colours, like a barcode no other element shares.
What's actually happening
It is tempting to think firework colours are some kind of coloured powder that burns brightly, but that is not it at all. The colour is the metal itself, glowing. When you pour energy into an atom by heating it, you can knock its electrons up onto higher rungs of an internal ladder. They don't stay there — almost instantly they drop back down, and each drop releases the surplus energy as a single particle of light. The size of the drop sets the colour: a big drop makes blue light, a smaller one red.
Here is the deep part. The rungs of that ladder sit at energies that are completely fixed and completely unique to each element. So when its electrons tumble back down, an atom can only ever emit its own specific colours — never a smooth rainbow, but a precise set of sharp lines, like a barcode. Sodium has a famous pair of lines in the yellow, which is why street lamps and a pinch of salt thrown in a flame both glow the same insistent orange-yellow. Strontium and lithium fall in the red, copper and barium in the green and blue-green, potassium in a pale violet. Firework makers are essentially picking metals off a colour menu: strontium for crimson, barium for green, copper for the prized and difficult blues.
That barcode of lines turned out to do far more than entertain. In the 1800s, scientists realised the dark and bright lines in the Sun's light were these same elemental fingerprints — and by reading them, they could say what the Sun was made of without ever leaving Earth. Helium was discovered this way, spotted as an unexplained yellow line in sunlight years before anyone found it on the ground. Today every claim about the composition of a distant star, a nebula, or the atmosphere of a planet around another sun rests on the very effect lighting up the sky on New Year's Eve.
Each metal glows its own exact set of colours, the same elemental fingerprint that lets us read what distant stars are made of.
- 1With an adult, dip a clean metal skewer in water, then in table salt, and hold it at the edge of a gas flame.
- 2The flame flares yellow-orange — that is sodium's emission lines, the same glow you see in a sodium street lamp.
- 3Rinse, dry, and try a pinch of a copper-based fungicide or a salt substitute (potassium): the flame shifts toward green or lilac. Each metal, its own colour.
Common questions
An atom's energy levels sit at fixed, unique spacings, so its electrons can only release specific amounts of energy as they fall. This produces a precise set of sharp lines, its emission spectrum, like a barcode no other element shares.
Copper makes firework blue, but the compounds break down at high temperature, so a clean deep blue is the rarest and most prized. Pyrotechnicians treat a good blue as a mark of real skill.
Reading these elemental fingerprints in starlight lets astronomers say what distant stars are made of. Helium was discovered this way, spotted as an unexplained yellow line in sunlight in 1868, before it was ever found on Earth.