Phosphorescence & Fluorescence (and why one “stops showing off” the moment you turn the lights on)

If you’ve ever waved a highlighter under a UV lamp and watched it practically shout neon, you’ve met fluorescence. If you’ve ever charged up a “glow-in-the-dark” sticker, turned the lights off, and it kept glowing like it’s refusing bedtime, you’ve met phosphorescence.

They’re cousins. They both involve electrons being excited and then falling back down, releasing light. The difference is how quickly they stop showing off.

Fluorescence: the instant response

Fluorescence happens when a substance absorbs higher-energy light (often UV) and almost immediately re-emits some of that energy as visible light.

Turn the UV on → it glows.
Turn the UV off → it stops (almost instantly).
Typical timescale: nanoseconds (that’s 0.000000001 s).

What’s happening (simple version):

Light energy bumps an electron to a higher energy level.
It loses a bit of energy as vibration/heat.
It drops back down and emits a photon (light) straight away.

Think of fluorescence as the student who answers immediately with their hand already in the air.

Phosphorescence: the “hang on, I’m still thinking” glow

Phosphorescence also starts with absorption of energy, but the excited electron gets “stuck” in a longer-lived state before it can return to normal.

Shine light → it charges up.
Turn the light off → it keeps glowing for seconds… minutes… sometimes hours.
Typical timescale: milliseconds to hours.

Why the delay?
The electron slips into a state called a triplet state that’s “forbidden” (in quantum rules terms) to drop back down quickly. It’s allowed eventually — just slowly.

Phosphorescence is the student who says, “I’ll email you the answer later,” and actually does.

Quick, safe classroom demos (with explanations)

1) Highlighter fluorescence (cheap, bright, reliable)

You need: UV torch or UV lamp (365–395 nm), yellow highlighter, paper, or a beaker of water.

Method A (paper):

Draw a thick line with a highlighter.
Shine UV on it. Instant glow.

Method B (solution):

Scribble on filter paper, soak in warm water, squeeze/filter into a beaker.
Shine UV: the beaker glows strongly.

Chemistry link: Many highlighters contain fluorescent dyes (often related to pyranine-type dyes). Great intro to absorption/emission and energy loss as heat (Stokes shift).

2) Tonic water fluorescence (the “party trick” that is actually chemistry)

You need: Tonic water, UV lamp, clear glass.

Tonic water contains quinine, which fluoresces blue under UV. It’s a lovely “invisible until UV” example.

Teach it as: “Same liquid, same room… different light source = different observation.” Great for discussing instrumentation and why UV-visible spectroscopy exists.

3) “Glow-in-the-dark” pigment (phosphorescence you can time)

You need: Glow powder/paint (commonly strontium aluminate-based), a bright lamp or UV light, a dark corner, stopwatch.

Charge it under bright light.
Turn lights off.
Time how long the glow remains clearly visible.

Stretch activity: Compare:

charged under normal LED light vs UV
warm vs cool conditions (careful and modest — don’t cook it!)
thin vs thick layer

Chemistry/physics link: Long-lived excited states and slow release of photons. Talk about energy traps/defects in the crystal lattice (age-appropriate version).

4) Glow sticks: chemiluminescence (bonus “third way”)

Glow sticks aren’t fluorescence or phosphorescence — they’re chemiluminescence (chemical energy → light).

Why include them?
Students often mix all “glowing” together. This is a brilliant chance to sort the zoo:

Fluorescence: needs a lamp shining right now
Phosphorescence: charges up then glows later
Chemiluminescence: glows from a chemical reaction

If you want one slide that saves a thousand misconceptions, it’s that.

The exam-friendly explanation (without the quantum headache)

Key comparison table (say it out loud like a spell)

Fluorescence: fast emission, stops when excitation stops
Phosphorescence: slow emission, continues after excitation stops
Both involve: absorption → excited electrons → light emitted on return
Difference comes from: whether the electron can return quickly (allowed) or gets stuck (forbidden/slow)

Vocabulary students should use confidently

Excitation
Emission
Photon
Energy levels
Stokes shift (for brighter groups)
Triplet state (A-level or keen GCSE extension)

Safety and good lab habits (the grown-up bit)

UV light: avoid shining into eyes; don’t encourage students to “test” it on their skin. Use UV safety glasses if you have them.
Keep UV exposure brief and controlled (teacher-held lamp/torch works well).
If using glow powders/paints: avoid dust inhalation; keep it off food/drinks; wash hands.

Wrap-up (with a teacher’s grin)

Fluorescence is the dramatic one: “LOOK AT ME!”
Phosphorescence is the stubborn one: “I’m still glowing, thanks.”
And glow sticks are the chaotic cousin who turns up uninvited and steals the show.

Once students can classify a glow correctly, they’ve basically learnt: energy transfer, electronic structure, and a slice of quantum rules — using a highlighter and a torch. That’s a good day in a school lab.

27 February 2026