PHYSICS WORTH SHARING
This weekend, a lot of porches have a glow-in-the-dark skeleton on them. By midnight it has faded. Here's a question that splits a room every time: if you try to recharge it with a red flashlight, will it glow again?
It won't. Shine a blue or UV light on it for a few seconds and it lights up green. Shine a red flashlight on the same skeleton, even a brighter one, and you get almost nothing.
A brighter red light should carry more energy. So why can't it wake the skeleton up?
Big idea
Light arrives in packets called photons, and each photon's energy depends only on its frequency: E = hf. Blue light has a higher frequency than red, so every blue photon carries more energy than every red photon.
The glowing paint on that skeleton is a phosphor, and it charges one photon at a time. Each absorbed photon has to lift an electron across an energy gap into a state where it can wait for a while before dropping back down and giving off light. A red photon carries less energy than that gap, so it can't make the jump, no matter how many of them arrive. Blue and UV photons carry more than enough. Making the red light brighter just sends more photons of the same small size.
Run the numbers with hc ≈ 1240 eV·nm. A red photon at about 650 nm carries roughly 1.9 eV. A blue photon at about 450 nm carries roughly 2.8 eV, and a UV photon at about 365 nm carries roughly 3.4 eV. The green glow itself, around 520 nm, is about 2.4 eV per photon. Energy conservation settles it: a red photon can't pay for a green photon that's worth more than it is. Blue and UV can, with some energy left over as heat.
That slow drop back down is what makes it phosphorescence instead of fluorescence. In fluorescence, the electron falls back almost instantly, so the glow stops when the light does. In phosphorescence, the electron sits in a longer-lived state, so the stored energy trickles out as light for minutes after the flashlight goes off. That's the skeleton glowing on the porch long after the porch light is off.
Ghost-writing with light (run it Monday)
For: Waves, the electromagnetic spectrum and photon energy. Time: 5 minutes. Materials: a glow-in-the-dark sheet, sticker or toy (a leftover glow skeleton or ghost works), a red LED flashlight, a blue LED flashlight or a low-power UV flashlight, and a room you can darken.
Ask: "If I write BOO with the red light and BOO with the blue light, which one will glow? What if the red one is brighter?" Have students commit to a prediction.
Dim the room. Write BOO on the sheet with the blue or UV light. Then write BOO beside it with the red light.
Turn the flashlights off. The blue BOO glows. The red BOO barely shows up, if at all. If the sheet was already glowing, the red letters may even look a little darker, because red light warms that spot and makes it give up its stored energy faster.
Land it: brightness is the number of photons, color is the size of each one. The sheet only responds to photons that are big enough.
Eye safety: never shine a UV light or any bright light into anyone's eyes, including your own. Use only a low-power UV flashlight, and only with adult supervision.
Summary
Each photon's energy is set by its frequency, E = hf. Blue photons carry more energy than red ones.
Glow-in-the-dark material absorbs one photon at a time, so a photon below the threshold does nothing, however many arrive.
The glow comes out at lower energy than the light that charged it, and it lingers because the excited electrons are slow to fall back.
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One quick question
Tell us what's next in your room and we'll build the next issue around it.
What unit are you teaching in the next 2 weeks?
Sources: American Physical Society, Energy of Light (PhysicsQuest teacher guide) and Hershberger, Sarquis and Hogue, Using LEDs to Demonstrate Properties of Light.
Forward this to a colleague who's teaching the electromagnetic spectrum this fall.
Stay Wildly Curious — Lauren & Shawn
