Meng Cui and Changhuei Yang demonstrated digital optical phase conjugation through a multiple scattering sample. A charge-coupled device measured a transmitted light field by wave interference with a reference. A calibrated spatial light modulator then imposed the opposite spatial wave phase on the returning beam. Comparing this return with a zero-phase control showed recovery of the original optical mode. The method separated field measurement from playback and made finite sampling and phase-only control explicit limits on reconstruction.^[ref]

An optical physicist and coauthor of the digital optical phase conjugation experiment described there.^[ref]

An experimental physicist known for measurements of electron charge and the photoelectric effect. The Millikan photoelectric experiment describes the apparatus and stopping-voltage measurements behind his 1916 test.^[ref]

Otto Robert Frisch and Lise Meitner interpreted the barium found after uranium bombardment as evidence that a heavy atomic nucleus could divide into two large fragments. Their 1939 paper related the available energy to nuclear mass differences and the electrical repulsion of the fragments. This supplied a physical account of nuclear fission, connecting a chemical identification of the products with a new type of nuclear transformation and its large energy release.^[ref]

A physicist who worked with Lise Meitner on the interpretation of nuclear fission.^[ref]

A physicist who helped explain nuclear fission. Lise Meitner and Otto Robert Frisch interpreted the energy release associated with the chemical findings of Otto Hahn and Fritz Strassmann. Following these contributions together distinguishes the identification of reaction products from their physical explanation.^[ref]

A physicist associated with the Bohr model of the chemical atom. His 1922 Nobel Prize in Physics concerned atomic structure and radiation.^[ref][ref]

Max Planck introduced energy elements proportional to frequency, $ϵ=h\nu$, in his statistical treatment of thermal resonators. This led to a mean resonator energy $\overline{E}=h\nu /(e^{h\nu /(k_{B}T)}-1)$ and the corresponding black-body radiation spectrum, connecting entropy counting with measured radiation. For $h\nu \ll k_{B}T$, expanding the exponential gives $\overline{E}\approx k_{B}T$; at high frequency the denominator suppresses the energy exponentially. The energy-element argument was a step toward quantum theory, without yet being the later light-quantum account of a free photon.^[ref]

A physicist whose work on black-body radiation led to Planck's law and the use of the Planck constant. He received the 1918 Nobel Prize in Physics.^[ref]

Paul Dirac sought a relativistic quantum mechanics whose evolution equation remained first order in both time and spatial derivatives. A free-particle Hamiltonian $H=c\mathbf{\alpha }\cdot \mathbf{p}+\beta m{c}^2$ reproduces the relativistic relation $H^2=c^2{\mathbf{p}}^2+m^2{c}^4$ when its coefficient matrices satisfy suitable anticommutator identities. The required multi-component wave function incorporates electron spin within the relativistic dynamics. His 1928 paper used this construction to address the doubling of atomic states that earlier theories had treated by adding spin separately.^[ref]

A physicist who developed a relativistic theory of the electron. He shared the Nobel Prize in Physics in 1933 with Erwin Schrödinger.^[ref]

A physicist who developed wave mechanics. In 1926, Erwin Schrödinger made the energies of chemical atoms an eigenvalue problem, rather than prescribing a separate quantization rule for each classical orbit. He shared the Nobel Prize in Physics in 1933 with Paul Dirac.^[ref]

The Schrödinger equation turns a proposed Hamiltonian operator into predictions for a wavefunction and its evolution. Boundary conditions can select discrete energy eigenvalues, while quantum superposition makes their relative quantum phases observable through interference. The infinite square well gives a transparent worked example; a hydrogen atom supplies a physical application with a different potential energy.

The Davisson–Germer experiment gives a complementary experimental route into matter waves: an accelerated electron beam scattered from a nickel crystal produces direction-dependent collected electric current. Its apparatus and angular scans make the wave prediction testable. Interpreting a wavefunction as a probability amplitude also requires the Born rule; that statistical interpretation deserves its own history alongside Erwin Schrödinger's dynamics.

A physicist whose work connects special relativity, general relativity, Brownian motion and the photoelectric effect. He worked at the Institute for Advanced Study from 1933 until his death in 1955.^[ref]

In 1905, Albert Einstein used the photoelectric effect to explore quantized light, related Brownian motion to molecular agitation, formulated special relativity, and connected changes in mass and energy. These problems link visible measurements to otherwise inaccessible structure: emitted electron energies, suspended-particle displacements and comparisons of clocks.^[ref]

The Millikan photoelectric experiment follows one of those predictions into the laboratory. Its stopping-voltage measurements test a quantitative relation; the experiment's apparatus, surface preparation and electrical corrections matter to that inference. The Einstein field equations and general relativity lead to a different set of tests involving gravity and spacetime.

The Nobel Prize in Physics in 1921 recognized his theoretical work, particularly the law of the photoelectric effect; the award was presented in 1922.^[ref]

Karl Bergwitz made early balloon measurements of atmospheric ionization, comparing the detector response at altitude with observations nearer the ground. These experiments tested expectations for radiation originating in the terrestrial environment.^[ref]

The experiment's value lies in changing the material between a proposed source and the detector. Interpretation also depends on instrumental behaviour under changing conditions, so altitude dependence must be separated from leakage, calibration changes and the detector's own background.

Alessandro De Angelis coauthored calculations of the transparency of intergalactic space to gamma rays. The work connects an assumed background photon population with energy-dependent optical depth and the attenuation of distant sources.^[ref]

High-energy photon propagation is affected by interactions with background radiation, including pair production. Interpreting a source's observed spectrum therefore requires both an emission model and a propagation model; a missing high-energy component need not originate entirely at the source.

Domenico Pacini tested the origin of atmospheric ionizing radiation by placing a sealed measuring instrument below the surface of water. His experiments compared the rate of discharge near the surface with that measured underwater.

A lower discharge rate underwater showed that part of the ionization came from radiation attenuated by the overlying water. Instrumental leakage and radiation from surrounding material also had to be considered. The result challenged explanations assigning all the penetrating radiation to local terrestrial sources, using shielding rather than altitude as the controlled comparison.^[ref]

Theodor Wulf developed a transportable electrometer for measuring atmospheric ionization. His design used fine fibres in place of delicate metal leaves, making outdoor measurements easier to calibrate. Comparisons at different heights, including the Eiffel Tower, tested whether radiation from the ground explained the ionization.^[ref]

The method infers ion production from loss of stored electric charge. Leakage and instrument stability must therefore be controlled before a change in discharge rate is interpreted as a change in penetrating radiation.

Ernest Marsden and Hans Geiger observed alpha particles scattered through large angles by metal targets. Counting weak scintillations supplied an event-based readout even when only a small fraction of the incident beam returned toward its source. The observation challenged descriptions that attributed all deflections to many weak encounters. Their later measurements varied geometry and target conditions to test the single-scattering interpretation behind Rutherford scattering.^[ref]

A physicist.^[ref]

Hans Geiger and Ernest Marsden counted large-angle deflections of alpha particles from metal targets. Their 1909 work used visible scintillations to register particles scattered back toward the incident side, establishing that rare strong deflections required explanation. Their 1913 measurements tested the angular dependence more systematically with thin targets. Comparing count rates with target thickness, incident energy and observation angle helped turn the concentrated-charge picture of Rutherford scattering into a quantitative experimental test.^[ref]

A physicist.^[ref]

Ernest Rutherford's 1911 calculation explained rare large-angle alpha particle scattering by concentrating an atom's central electric charge into a region small compared with the atom. In the single-Rutherford scattering approximation, the predicted differential scattering cross-section varies as ${\sin }^{-4}(\theta /2)$ and as the inverse square of the incident kinetic energy. These dependencies make the model testable through angular counts and controlled changes of beam energy, beyond merely observing that occasional particles turn backward.^[ref]

A physicist.^[ref]

The Rutherford scattering model connects large-angle deflections to concentrated electric charge. The Geiger-Marsden experiment supplies the experimental context; a predicted angular distribution needs comparison with detector geometry and background counts.

Heike Kamerlingh Onnes liquefied helium, opening access to temperatures far below those reached with other common cryogenic liquids. This enabled electrical measurements through the low-temperature transition of metals and the discovery of superconductivity. His later liquid-helium experiments also investigated persistent currents, connecting the sharp resistance change with sustained electrical behavior in a closed circuit. The progression shows how a new cooling technique can expose phenomena that are inaccessible at higher temperatures.^[ref][ref]

A physicist.^[ref]

Helium liquefaction supplied a route to very low temperatures. The resistance-versus-temperature experiment for superconductivity and measurements of superconducting persistent current show how reaching those conditions opened new questions about electrical behavior.

Peter Debye explained the heat capacity of a solid by counting a spectrum of quantized elastic vibrations. In the Debye model, a cutoff ensures the correct number of modes, while the low-frequency acoustic spectrum gives heat capacity proportional to $T^3$ at sufficiently low temperature. At high temperature, the same construction approaches the classical Dulong-Petit law. Connecting elastic vibrations with quantum occupation explains why a solid stores much less thermal energy at low temperature than a classical mode count predicts.^[ref]

A physicist.^[ref]

An experimental condensed matter physicist who developed a low-temperature relaxation calorimeter for small samples. A sapphire platform is connected to a cold bath by a controlled thermal link; its temperature relaxation determines heat capacity. Explaining the platform's own thermal resistances is essential before interpreting a sample's small signal as a material property.^[ref]

An experimental physicist who coauthored the observation of X-ray diffraction by crystals. A photographic plate recorded direction-dependent spots from radiation transmitted through a crystal, linking the periodic arrangement of matter to interference of short-wavelength waves. The result made crystalline structure accessible through the geometry of a diffraction pattern.^[ref]