Franck–Hertz experiment – excitation, emission and ionisation
PhysicsQuantum & Nuclear PhysicsAges 17–18Virtual lab
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Use it with your class: students open it from a link or a QR code, with no account needed.
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Sweep the voltage across a mercury or neon tube and plot collector current: it falls each time electrons have gained enough energy to excite an atom. Mark the maxima and find the excitation energy from their spacing. Further screens fire single electrons at an atom and show how a fluorescent tube turns electron energy into UV and then visible light.
Lesson: Excitation and ionisation by electron collisions, energy levels, the Franck–Hertz experiment, the fluorescent tube
What it shows
In 1914 Franck and Hertz fired electrons through mercury vapour. The collector current rises with voltage, then falls each time electrons have gained about 4.9 eV, because an atom accepts that exact amount of energy and no less. Equal steps between maxima show that atoms have separate energy levels. The excited atom gives the energy back as an ultraviolet photon near 254 nm. A fluorescent tube uses the same collisions, with a coating that turns the UV into visible light. The model gives equal spacing, and its current scale is illustrative.
How to use
On Tube, press Sweep to plot the current against voltage. Drag on the graph, or use Accelerating voltage, to reach a maximum and press Mark maximum. Mark at least three and read the mean spacing. Change Oven temperature, Retarding voltage or Gas and sweep again. On Atom and electron, set Electron energy and press Fire electron. On Fluorescent tube, choose a Coating, set Electron energy at collision and press Record data.
Parameters you can change
- Screen Tube, Atom and electron, Fluorescent tube
- Gas Mercury, Neon
- Accelerating voltage 0–80 V
- Oven temperature 100–200 °C
- Retarding voltage 0–3 V
Questions to explore
- What spacing do you measure between the mercury maxima, and what photon wavelength does that energy give?
- How does the curve change when you cool the oven to 110 °C, and why does the current then climb?
- What share of a UV photon's energy stays in visible light with each coating, and where does the rest go?
