Definition
Chemiluminescence is what happens when substances mix and react, and part of the reaction's energy comes out directly as light instead of hot heat. You don't plug anything in or light a match. The chemical reaction itself makes the light.
What happens when you snap a glow stick?
Picture the glow sticks people wave at festivals and concerts. You bend the plastic tube until it snaps with a little "crack," give it a shake, and a bright glow blooms. Strangely, it never gets hot enough to burn your hand.
Inside the tube is a thin glass vial holding one liquid, and around it is a different liquid kept separate. When you bend the stick hard, the inner glass breaks. The two liquids mix and a real chemical reaction begins.
When you burn a log or a candle, the reaction gives off a lot of heat. But the chemicals in a glow stick are designed to send part of the reaction's energy out as light particles called photons instead of heat. That's how you get "cold light" that feels cool to the touch.
A closer look: excited electrons hop back down
To be more precise, chemiluminescence comes from a change in the energy state of electrons inside molecules. When the two chemicals react, the new molecule is made with an electron already perched on a high step of a staircase. (In a glow stick, this energy gets passed on to a fluorescent dye molecule.) This is called an "excited state."
But an electron on a high step is very unstable, and it can't stay there for long. In the blink of an eye, it slides back down to its comfortable home step, the ground state, and sheds the extra energy as it goes.
That extra energy is the visible light we see. Most chemical reactions waste this energy as heat, with molecules jiggling and bumping into each other. Glowing chemicals turn some of it straight into a beautiful glow instead. The rest still escapes as heat, but not enough for your hand to feel.
From crime scenes to fireflies
Chemiluminescence is a big help in forensic science, where investigators hunt for clues the human eye can't see. A famous example is the luminol test, which finds blood stains that have been wiped away at a dark crime scene.
When luminol solution is sprayed evenly over a wall or floor, the iron in hemoglobin, hidden in the blood, acts as a catalyst that helps the reaction along. Anywhere blood once touched lights up and reveals itself with a mysterious blue glow in the dark.
Nature uses this trick too. Fireflies lighting up the night sky make their own light. Some deep-sea fish make it themselves, and others get it from glowing bacteria living in their bodies. This kind of bioluminescence is a type of chemiluminescence driven by enzymes. Using light to attract a mate or snag a meal is a clever survival strategy.
Common misconceptions
Glow-in-the-dark stickers that shine at night after sitting in the sun are chemiluminescent too.
Glow-in-the-dark stickers and most glow paints on watch dials work by phosphorescence. They soak up light from outside and slowly release it. Chemiluminescence is a completely different process: no outside light is needed, because a new chemical reaction makes the light itself.
Where you meet it
Snapping a glow stick (chemical light) at a concert and seeing it instantly burst into glowing light
Crime scene investigators, like those on CSI, spraying luminol in a dark room to find tiny blood stains invisible to the naked eye as a blue glow
A firefly oxidizing a compound called luciferin in its abdomen to flash yellow light, a form of bioluminescence
In one sentence
Chemiluminescence is when part of the energy from a chemical reaction is released directly as light instead of heat.