# Explain how the Franck–Hertz experiment provides evidence for the existence of discrete energy levels in atoms.

> IB DP Physics Higher Level (2023 syllabus) — E.1 Structure of the atom · Explain · 4 marks

> In the Franck–Hertz experiment, electrons are accelerated through mercury vapour. The current collected at the anode is observed to drop sharply at regular intervals of approximately 4.9 eV as the accelerating voltage is increased.

## Mark scheme (4 marks)

1. Electrons collide with mercury atoms and can only transfer energy when they possess at least 4.9 eV (the energy difference between the ground state and first excited state).
2. Below 4.9 eV, collisions between electrons and mercury atoms are elastic, so electrons retain their kinetic energy and reach the anode, giving a steady (or rising) current.
3. At multiples of 4.9 eV, electrons undergo inelastic collisions, losing (almost) all their kinetic energy to the mercury atoms, and can no longer overcome the small retarding potential at the anode, causing the current to drop sharply.
4. The regular, repeating pattern of current drops at equal voltage intervals shows that only specific, discrete amounts of energy can be absorbed by the atom, which is consistent with quantised (discrete) energy levels.

## Key terms

- [discrete energy levels](https://www.gradenine.co.uk/glossary/discrete-energy-levels)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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