Above: Image © fir0002, Wikimedia

Stovetop burners, fireworks, and old-fashioned light bulbs all work because of dancing electrons.

A stovetop burner turns red when it’s set to maximum. An old-fashioned light bulb gets too hot to handle after it’s on for a while. Certain chemicals, when set on fire, produce brilliant colours. But why do heat and light always seem to go together?

The answer lies in the smallest unit of matter, the atom. The atom can be thought of as a miniature solar system, with the positively charged protons and uncharged neutrons in the centre, or nucleus, and the negatively charged electrons filling up “orbits” around it.

Did you know? Heat, light and electricity can all be used to move electrons between orbitals.

The electrons are what allow atoms to react with each other in chemical reactions, and they are also what cause certain compounds to emit light when heated. When an atom is heated up, it starts to vibrate intensely. Any object in motion is said to have kinetic energy and some of this energy is absorbed by the electrons around the nucleus, which they use to jump from their usual orbit into a higher orbit. When an atom has electrons out of place like this, it is said to be in an “excited state”. This excited state is very unstable, and the electron quickly falls back down (“relaxes”) to its normal orbit, releasing its stored heat energy in the form of photons, which we see as light.

Did you know? The tungsten filament in a standard incandescent light bulb heats up to over 3,000 degrees Celsius when in use!

This effect can be used in lots of cool ways! The most famous example is an incandescent light bulb. Here, an electrical current is forced through a small metal resistor (the filament). Resistors get hot when electricity is moved through them; so hot, in fact, that it causes the filament to glow. One of the reasons incandescent bulbs are being phased out is because the majority of the energy used – up to 95 per cent – is spent heating up the filament, rather than producing light!

Did you know? Colour production in fireworks is a fine art, and firework technicians often guard their new colour formulations closely! Some common ones: lithium carbonate burns red, barium chloride burns green, and copper chloride burns blue.

Fireworks also work on the same principle. Different salts (compounds containing metals) are used in the development of fireworks to produce different colours. This is because the amount of energy needed to rearrange electrons into a higher orbit depends on the type of metal contained in the salt. Metals that absorb higher energies will release shorter wavelength light (e.g. blue) when their electrons return to a lower orbit. Those absorbing lower energy will release light of longer wavelengths (e.g. red).

So there you have it — seeing light coming from a heated metal is really watching dancing electrons relaxing!

Learn more!

Derek van Pel

I am working on a Ph.D in Molecular Biology at the University of British Columbia, where my research is aimed at finding new anti-cancer therapeutics that can target weaknesses specific to tumour cells. I am an avid science educator, and when I'm not in the lab, writing for CurioCity, or volunteering with Let's Talk Science, I can be found teaching a 3rd year undergraduate biochemistry lab!

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Avatar  Joy

hi, if the characteristic light is from the cause of change in arrangement of electrons,excited from heat, can protons or neutron get excited by heat and produce characteristic light?