Radioactive things always glow green in movies or cartoons. Maybe if the art team was feeling a little creative there’s some blue in there. As a catchall glowing in a lab is typically shorthand for “this is radioactive,” and if you’ve never put thought into the radioactivity of things you probably wouldn’t be blamed for thinking there’s some merit to this Hollywood shorthand. Now you’re thinking about it though. Do radioactive things actually glow?
What Is Radioactivity Anyway?
You might be wondering what makes something radioactive in the first place. Radioactive decay is what happens when an atomic nucleus (the middle bit with protons and neutrons) loses energy. If the nucleus is unstable, it loses energy and is therefore considered radioactive. Radioactivity takes multiple forms–but it’s all centered around the ejection of particles from the nucleus. While radioactivity is super complicated (nuclear physics is its own field after all), radioactive decay can be best understood as “an unstable atom becoming more stable.”
When atoms shed protons and neutrons to become more stable, these particles become ionizing radiation. For us laypeople, this is just “the stuff you shouldn’t stand near for prolonged periods because then you will die.”
In case you were wondering, the most common types of radioactive decay are alpha, beta, and gamma radiation. The energy emitted from this kind of radiation is the stuff known for penetrating through walls and the like. There’s also proton and neutron emission, where the nucleus spits out either a proton or a neutron respectively. Spontaneous fission is when an atom just splits itself into two smaller atoms. Finally there’s electron capture and isomeric transition. The first one is when the nucleus takes an electron from somewhere else and spits out a neutrino. Isomeric transition is the release of gamma rays.
But does this energy glow?
Well. Kind of. Sometimes radioactive decay can produce photons, which means they can produce light. But radioactive decay produces light that isn’t part of the visible spectrum, so your eyes aren’t going to be picking it up.
Some radioactive elements like plutonium appear to glow, but plutonium does not glow red because of the ionizing radiation. It reacts with the elements in the air to create that glow. That’s a long way of saying plutonium gets super hot in air. That’s kind of the crux of how all glowing radioactive things produce a visible glow. There’s something else happening that makes the radioactive decay visible (if you could straight up see ionizing radiation, you’d see the UV rays of the Sun or the radiation used by X-ray machines).
While green is the common color for “things that are radioactive,” if you looked into a nuclear reactor you’d more likely see a blue glow. This is called Cherenkov radiation, and it’s a type of electromagnetic radiation. The glow is thanks to the electrons moving in water faster than light would in the same water.
Why green?
The classic radioactive “green glow” likely owes itself to radium. This glow is most commonly recognizable in glowing watch faces or gun sights, and tritium may also be used to achieve a similar effect. Nowadays most consumer products like watches that once used radium or tritium are photoluminescent instead. That’s the glow-in-the-dark stuff that glows after you sit it in front of a light for a while.
But again, radium itself doesn’t just glow. It doesn’t even glow green. A pure hunk of radium will glow blue, because the ionizing radiation reacts with the air. It doesn’t even glow that visibly.
But watches are green, right? That’s true, and it’s because radium was used in the paint coating them. That paint contains a phosphor (commonly zinc sulfide) that reacts with the radiation emitted by radium for that characteristic green glow.
Speaking of radiation, see if you know what sits on the electromagnetic spectrum here.
