Irregular Webcomic! #3319

Oooh. Aaah.

I should be home from my trip to San Francisco when this annotation goes live, however, I'm still going to be jetlagged, so I'm preparing this one before I leave.

I saw some fireworks last night. Being summer, this is cruise ship season, and several cruise ships spend a night in Sydney Harbour every week. They often put on fireworks displays for the passengers. And yesterday a friend of mine contacted me and suggested we head down to a lookout spot on the shore and take some photos.

Fireworks are a cool application of chemistry and physics. Specifically, the generation of light caused by excited electrons dropping down to less energetic states in the electron shells of atoms. I talked a bit about this in this previous annotation. To recap: Electrons are bound to atoms in so-called orbitals of specific energy levels, defined by the interactions of quantum mechanics. When an atom gains energy, by being heated, or physically collided, or having an electric current imposed on it, some of the electrons can either be stripped away, ionising the atom, or can be raised to higher energy level orbitals. When the electrons fall back to the lower energy level orbitals, they release the difference in energy as photons of light. And because the difference in energy is defined by the available energy levels, it occurs only with very specific quantities. These amounts of energy translate into specific frequencies (or equivalently, wavelengths) of light of the emitted photons, by Planck's relation: E = hν. (Energy equals Planck's constant times the frequency.)

One result of this is that if you excite atoms of an element by, for example, burning them in a flame, they give off specific and characteristic frequencies of light. We perceive this as specific colours of light. This is used routinely in chemistry labs as one of the tools used in identifying chemical elements and compounds. The test quite simply involves taking a small sample of the unknown compound or element, burning it in a flame, and observing what colour the flame turns. This is known as a flame test.

Left: copper statue covered in verdigris (the Quadriga on top of the Brandenburg Gate). Right: Clean copper statue (Portlandia, Portland).

For example, copper compounds burn a distinct blue-green colour. Similar to the colour of verdigris, in fact, which is corroded copper^[1]. Strontium compounds give a bright orange-red colour. Sodium gives bright yellow. Caesium gives a pure blue. Potassium and rubidium give violet shades. (Other elements are used too - I won't list them all.)

If you put chemical salts containing these elements into fireworks, then when the firework explodes, they generate the associated colours. And this is how pyrotechnicians do it. The beautiful colours you see in a fireworks display are the result of a carefully choreographed concoction of chemistry. Next time you see a fireworks display, don't just "ooh" and "aah" over its beauty - be thrilled and awed that when you know enough about physics and chemistry that you can understand and really appreciate what went into making such a display possible.

[1] Verdigris gives the Statue of Liberty in New York its distinctive green colour. It wasn't always like that. When dedicated in 1886, the Statue was a dark brown, the colour of copper, like a well-used copper coin. It remained that way up until about 1900, when the corrosion became noticeable. Over the following few years, the verdigris advanced quickly over the statue, coating it green. The US Congress, horrified, decided it had to be cleaned up and the exterior painted to protect it from such ravages. However, the public protested against painting the statue, and inspection revealed that the verdigris was not structurally damaging, so eventually they decided to just leave it. Now we're used to thinking of it as green.

Imagine if it was cleaned up and restored to its original brown colour. The colour would look something like the giant statue Portlandia, in Portland, Oregon. Certainly not unattractive, but definitely a change from what we are used to.