Franck–Hertz experiment

The Franck–Hertz experiment showed that electrons lose energy in discrete amounts when colliding with mercury atoms, providing evidence for quantized atomic energy levels.

Franck–Hertz apparatus: An electron tube with a heated cathode, accelerating grid, mercury vapor, and collecting electrode. Its electrodes accelerate electrons and measure which retain enough energy to reach the collector.
Electron: A negatively charged elementary particle found in atoms and forming electric currents. The experiment varies electron kinetic energy and observes how electrons scatter from mercury.
James Franck: A German physicist whose work on electron collisions with atoms earned the 1925 Nobel Prize in Physics. Franck conducted the experiments that revealed discrete energy transfer in mercury vapor.
Quantum mechanics: The physical theory describing matter and radiation through quantum states and probabilities. The experiment’s discrete collision energies anticipated a central feature of quantum descriptions of atoms.
Millikan oil-drop experiment: An experiment that measured the elementary electric charge using charged oil droplets. It established charge quantization through droplets, whereas Franck–Hertz measured discrete atomic excitation.
Inelastic collision: A collision in which kinetic energy is transferred into internal energy or other forms. Electrons surrender a fixed amount of kinetic energy when they excite mercury atoms.
Atomic energy level: A discrete allowed energy state of an atom’s electrons. Mercury absorbs collision energy only when it can reach a higher allowed state.
Gustav Hertz: A German physicist who studied electron collisions with atoms and shared the 1925 Nobel Prize in Physics. Hertz collaborated with Franck on the experiments and their interpretation.
Atomic spectroscopy: The study of atoms through their absorption and emission of electromagnetic radiation. It measures atomic energy differences through collisions rather than emitted or absorbed light.
Atomic emission spectrum: The pattern of wavelengths emitted by atoms as they transition to lower energy states. Spectral lines reveal energy differences through photons, unlike Franck–Hertz’s electron collisions.

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