Experiment Index

Choose an experiment by the question it answers: discrete atomic energies, the momentum of light, interference of matter, or correlations between separated systems. The dates below identify selected measurements or publications. The explanation column uses modern language, which often differs from the original interpretation.

Each experiment name links to its apparatus, reasoning, and limitations. The primary-papers index supplies original sources; the experiment-to-concept map compares the larger arguments.

Radiation, atomic structure, and discrete energies

Experiment and landmark period	What was measured	What the result helps explain
Thermal radiation, late 1890s–1900	The equilibrium spectrum as a function of wavelength and temperature.	The classical equipartition prediction fails at high frequency. Planck’s law dates from 1900; its detailed journal treatment appeared in 1901.
Photoelectric effect, 1887 onward; precision tests in 1916	Electron emission and stopping potential as the light frequency changes.	Maximum electron energy follows a frequency-dependent law. Einstein’s 1905 proposal was an explanation, not the first observation of photoemission.
Atomic line spectra, nineteenth century onward	Sharp emission and absorption frequencies, including the Balmer series described in 1885.	In the quantum explanation, lines correspond to energy differences. A list of lines alone does not determine a complete atomic model.
Geiger–Marsden scattering and Rutherford’s model, 1909–1911	A small fraction of alpha particles scattered through large angles.	Concentrated charge can account for strong deflections. Rutherford’s nuclear model is compatible with classical scattering and does not itself establish energy quantization.
Franck–Hertz collisions, 1914	Repeated reductions of collector current as electron accelerating voltage increases in mercury vapor.	Electrons lose energy in discrete atomic excitations. The original ionization interpretation must be distinguished from the later excitation account.
Stern–Gerlach, 1922	A neutral silver-atom beam split in an inhomogeneous magnetic field.	Discrete magnetic-moment projections; the modern account of the principal splitting uses electron spin, introduced later.
Compton scattering, 1923	An angle-dependent shift of scattered X-ray wavelength.	Energy and momentum transfer agree with photon–electron collision kinematics; see photon momentum.

Matter-wave diffraction and interference

Experiment and publication year	What was measured	What to compare
Davisson–Germer, 1927	Angle-dependent electron scattering from a nickel crystal.	Diffraction peaks and the wavelength $\lambda=h/p$.
G. P. Thomson and Alexander Reid, 1927	Diffraction rings after electrons passed through a thin film.	Transmission geometry, compared with the reflected/scattered beam in Davisson–Germer.
Jönsson’s electron slits, 1961	Electron interference using fabricated narrow slits.	The experimental slit geometry and coherent alternatives.
Tonomura and collaborators, 1989	A fringe pattern built up from individual electron detections in a biprism interferometer.	Localized detections and an interference distribution. This apparatus did not use two physical slits.

Both crystal diffraction and two-path interference require phase information. The probability-amplitudes page explains why coherent alternatives are added before taking the squared modulus. No individual dot on a detector is a miniature fringe pattern.

Correlations and collective quantum effects

Experiment and landmark year	What was measured	Next explanation
Freedman–Clauser, 1972; Aspect experiments, 1982	Polarization correlations violating Bell inequalities under the experiments’ sampling and apparatus assumptions.	Bell’s theorem and the difference between an inequality and its experimental test.
Bell tests closing the major locality and detection loopholes, 2015	Correlations of separated spins or entangled photons, with improved detection and spacetime arrangements.	Which assumptions remain, including statistical independence of settings. “Loophole-free” does not mean assumption-free.
Integer quantum Hall discovery, 1980	Hall-resistance plateaus in a two-dimensional electron gas in a strong magnetic field.	Integer quantum Hall effect. Distinguish this result from the fractional effect discovered later.
Dilute-gas Bose–Einstein condensation, 1995	A narrow component in the expanded atomic cloud’s distribution, with behavior characteristic of condensation.	Bose–Einstein condensation and the role of quantum statistics, trapping, and interactions.

Connecting two measurements of an energy gap

An electron accelerated through a potential difference $V$ gains energy $eV$, where $e$ is the positive elementary charge. A loss near $4.9\,\mathrm{eV}$ in mercury collisions and an emitted ultraviolet photon near $254\,\mathrm{nm}$ can be compared through

$$\Delta E=h\nu=\frac{hc}{\lambda}.$$

This agreement connects collision measurements with spectroscopy. It does not require a classical electron orbit, nor does it identify the excitation energy with the larger energy needed to ionize the atom. Contact potentials and the electron energy distribution matter when interpreting actual current–voltage curves; the Franck–Hertz page develops that qualification.

Exercise

Using $hc\simeq1240\,\mathrm{eV\,nm}$, estimate the photon wavelength associated with a $4.9\,\mathrm{eV}$ atomic excitation. Why is agreement with an observed spectral line a stronger check than reproducing either measurement alone?

Solution

The wavelength is $1240/4.9\simeq253\,\mathrm{nm}$, consistent at this precision with the mercury line near $254\,\mathrm{nm}$. Collision energy losses and photon frequencies are measured by different methods. Their agreement supports a common atomic energy difference, while making a device-specific explanation of only one measurement less plausible. It does not by itself establish all the postulates of quantum mechanics.

References

The linked experiment pages contain fuller bibliographies. These sources document the distinctions emphasized here.

• M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Wieman, and E. A. Cornell, “Observation of Bose-Einstein Condensation in a Dilute Atomic Vapor,” Science 269, 198–201 (1995), doi:10.1126/science.269.5221.198.
• J. Franck, “Transformation of Kinetic Energy of Free Electrons into Excitation Energy of Atoms by Impacts”, Nobel lecture, 11 December 1926 — a later account of the collision experiments and their interpretation.
• W. Gerlach and O. Stern, “Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld,” Zeitschrift für Physik 9, 349–352 (1922), doi:10.1007/BF01326983.
• B. Hensen et al., “Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres,” Nature 526, 682–686 (2015), doi:10.1038/nature15759.
• K. von Klitzing, G. Dorda, and M. Pepper, “New Method for High-Accuracy Determination of the Fine-Structure Constant Based on Quantized Hall Resistance,” Physical Review Letters 45, 494–497 (1980), doi:10.1103/PhysRevLett.45.494.
• G. P. Thomson and A. Reid, “Diffraction of Cathode Rays by a Thin Film,” Nature 119, 890 (1927), doi:10.1038/119890a0.
• A. Tonomura, J. Endo, T. Matsuda, T. Kawasaki, and H. Ezawa, “Demonstration of single-electron buildup of an interference pattern,” American Journal of Physics 57, 117–120 (1989), doi:10.1119/1.16104.