The quantum origins of glow-in-the-dark

I grew up with glow-in-the-dark stars pinned to my bedroom walls. I stuck some on my ceiling fan too, so that I’d have ‘shooting stars’ to spin over my bed. But I never paid attention to how they worked. No battery, no heat. Just a haunting pale glow.

Like most things in life, if you dig deep enough, you’ll find that it’s quantum all the way down.

The story of phosphorescence – the word for glow-in-the-dark when you’re feeling fancy – begins 147 million kilometres away, as a photon leaps off the Sun. Eight minutes later, it barrels into Earth’s atmosphere. Some microseconds after that, it hits something solid. Some opaque objects, like a piece of shale or concrete, will absorb the energy of the photon and may even heat up – attracting turtles and other reptiles to bask on warm rocks on sunny days. But that’s not the story we’re telling here.

We’re interested in what happens when it hits materials that glow.

 

 

 

 

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We’ve just described fluorescence, not phosphorescence. In fluorescence, the glow goes away as soon as the light source is removed. The absorbed photon gets re-emitted mere nanoseconds later – almost instantaneously to our eyes.

So let’s get quantum for a moment. All materials are made up of particles like electrons and protons bound up inside atoms. These particles can gain or lose energy – for instance by being struck by an incoming photon – but their energy doesn’t change on a smooth scale. Try not to think of it like an inclined ramp to roll up or down. It’s more like a ladder. A particle’s energy can take a full step up, but it can’t step up only half a rung. It’s all or nothing.

In some materials – safety vests, highlighters, and so on – an incoming photon will be absorbed by an electron in that material, causing it to jump up a rung into a higher energy state. But it doesn’t last very long. As particles are jostled and bumped around in the normal vibrations of the world at a particle scale, that energy will be lost. And lost quickly. The photon gets re-emitted back out at a lower wavelength, while the excited electron falls back down to a lower rung again.

Fluorescence is still a neat phenomenon. The incoming photon itself underwent a change in this process. It lost some energy, and changed its wavelength (and therefore its colour). High energy, short-wavelength photons (like ultraviolet, blue, and purple light) will come out again with longer, lower-energy wavelengths, like orange or red. So a safety vest, for example, collects invisible ultraviolet light and releases it again at wavelengths we can see, making it look shiny and bright to our eyes when the Sun hits it.

But other materials can hold onto the photon longer. They keep it even as the darkness encroaches. This is phosphorescence.

There are two main mechanisms behind phosphorescence. ‘Triplet phosphorescence’ is a tricky one – we’ll come back to that later. But the one we really care about is called ‘persistent phosphorescence.’ It only works where there are plenty of atoms in a material all strung together like a lattice or crystal. A single atom won’t cut it. In these scenarios, there can sometimes be gaps in the crystalline structure of the lattice – a missing atom in the material, a defect. But that defect makes the magic happen – the incoming photon gets trapped in the lattice, filling the hole, and making the material glow.

It won’t last forever. Eventually, vibrations will knock the photon free, and it will sail off at a lower wavelength just like it did with fluorescence.

And here’s the coolest bit (no pun intended). If you cool down a phosphorescent material, you can make the glow last longer, because it reduces the vibrations that might knock the photon free.

Even cooler – you can turn a fluorescent material into a phosphorescent one by cooling it down too, delaying the vibrations that kick the photon away.

So that’s it. That’s the quantum origin of my bedroom’s glow-in-the-dark stars.

We’re done here, unless you really want to dig deeper. In which case, there’s:

Triplet phosphorescence

Triplet phosphorescence also involves a photon being captured and released, but the mechanism for trapping the photon’s energy and delaying its escape is different.

Just like the ladder rung rule, another fundamental law of quantum mechanics says that multiple electrons in an atom cannot occupy the same state in the same place at the same time. It’s called the Pauli Exclusion Principle. This rule is one of the reasons you can’t push your hand through a wall like a ghost.

To get specific, the exclusion principle means you can only have two electrons in the same orbital around an atom at a time, and those two must have opposite spins.

When an incoming photon knocks one of those electrons up into a higher energy orbital, there is a chance – a low probability, but a chance – that its spin will flip directions, no longer constrained by its previous partner to remain the opposite spin. But this is rare. It’s far more common for the excited electron to just keep its existing spin as it jumps. It is also rarer in reverse too, for that electron to flip its spin to fall back down. And that’s the key here. The flipped spin electron can stay in the excited state longer merely by the laws of probability – it takes longer for the electron to flip spin and fall back down. So it phosphoresces!

What about fireflies and bioluminescence?

Some glow-in-the-dark occurrences are chemical in nature. Bioluminescence in insects, or a ‘glow stick’ you crack open to start – these are initiated by chemical reactions between two compounds. 

The quantum processes are similar, but the trigger is different. And they make some of the most beautiful displays you can see in nature.

Are you a teacher? Want curriculum-friendly lesson plans for quantum processes like those in this article? Check out our teacher resources page.