Franck–Hertz tube (3D)
About this app
The Franck–Hertz experiment in 3D: drag the voltage, the curve draws live and the current drops at equal intervals. Measure the minima, calculate energy and wavelength, with mercury or neon and its glowing layers.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
What the Franck–Hertz experiment shows
Electrons travel from a hot cathode through mercury vapour to a grid and gain energy on the way. Below 4.9 eV they collide elastically with the atoms and lose practically no energy. From 4.9 eV they give exactly this amount to an atom. Behind the grid a small retarding voltage brakes them, so these electrons are missing at the collector and the current drops. This happens at equal voltage intervals. In 1914 James Franck and Gustav Hertz showed in this way that atoms take up energy only in fixed portions.
What the app shows
The tube is at the centre, with cathode, grid, collector, atoms and flying electrons. Collision zones glow where electrons reach the excitation energy. Below the tube an energy band shows the energy of an electron at every position as a sawtooth. The curve I(U) draws live while you drag the voltage. With neon, orange-red glowing layers appear, and more of them form at higher voltages.
What you can try
Predict before the first dip how the current will continue. Mark the minima, find the spacing and calculate energy and wavelength from it. Follow a single electron in slow motion and change the retarding voltage. In the Atom panel you set the energy of a test electron and see that the atom takes up nothing or exactly 4.9 eV. Exam tasks with worked solutions check your calculations.
Frequently asked questions
- Why does the current drop in the Franck–Hertz experiment?
- As soon as electrons reach the excitation energy just before the grid, they give it to an atom. Their energy is then too small to reach the collector against the retarding voltage. As the voltage rises the collision zone moves towards the cathode, the electrons gain energy again and the current rises once more.
- Why is the first minimum not at 4.9 V?
- The contact potential between cathode and grid shifts the curve by about 2 V, depending on the tube. What counts is therefore the spacing of two minima: for mercury it is 4.9 V, and this shift does not change it.
- Why does neon glow visibly and mercury not?
- Mercury gives off its 4.9 eV in one step as a UV photon at 253.7 nm. Neon is excited to about 18.7 eV and falls back in two steps. The first step of about 2 eV lies in the visible range and produces the orange-red glow.
- What can I measure in the app?
- You mark the minima of the curve. The app calculates the mean spacing, the energy in eV and joules and the wavelength. The current values are model values, the position of the minima follows the measured energies.
Subject: School physics
More from School physics
- The exposure triangle
- Density and buoyancy
- The Doppler effect
- Additive and subtractive colour mixing
- Photoelectric effect (3D lab)
- Newton’s cradle and collisions
- Particle in a box (3D)
- Converging lenses and ray diagrams
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