What Causes Hot Things to Glow?

A stovetop burner glows 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 seem to go together?

The answer lies in the smallest unit of matter, the atom. At the centre of the atom is the nucleus, which contains positively-charged protons and uncharged neutrons. Negatively-charged electrons exist in shells, which are the areas that surround the nucleus. A shell is also sometimes called an energy level.

How can heat be transformed into light?

Electrons are what allow atoms to react with each other in chemical reactions. They are also what cause certain compounds to emit light when heated. 

Each electron has a shell that is its “home base.” When it is in that shell, it is said to be in its ground state. The ground state is the state of lowest energy for that electron. At room temperature, most electrons are in their ground state.

When heat is transferred to an atom, it starts to vibrate more quickly. This vibration is a form of kinetic energy. Some of the kinetic energy is transferred to the electrons around the nucleus. This makes them “jump” from their usual shell into a shell that is further away from the nucleus. When an atom’s electrons move 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 to its normal shell, and ground state. When this happens, the electron releases the extra energy it had gained in the form of infrared light photons. These photons are invisible to the human eye.

How do light bulbs make light?

The ability of electrons to convert heat to infrared light can be used in lots of cool ways! The most famous example is an incandescent light bulb. Inside the glass bulb is small piece of metal (filament) made of tungsten. An electrical current is passed through the filament. But it doesn’t pass through easily. This is because the filament acts as a resistor. Resistors get hot when electricity passes through them. So hot, in fact, that it gives off enough energy to cause the filament to glow. This is the same reason that the cooking elements on an electric stove glow red when they heat up.

Did you know? 

The tungsten filament in an incandescent light bulb heats up to over 2 000 degrees Celsius when in use!

One of the reasons incandescent bulbs are being phased out is because the majority of the energy used – up to 95 percent – is spent heating up the filament, rather than producing light. That is a waste of energy!

How do fireworks make light?

Fireworks also work on the same principle. Fireworks are made from different compounds containing metals. When heated, these compounds produce different colours of light. Metals that absorb higher energies will release shorter wavelengths of light (such as blue light) when their electrons return to their ground state. Metals that absorb lower energies will release longer wavelengths of light (such as red light) when their electrons return to their ground state.

Did you know? 

Colour production in fireworks is a fine art, and fireworks technicians often guard their colour recipes very closely! 

So there you have it. Heat and light go hand in hand to create some amazing things. From lighting up our homes to lighting up the night!