Walther Friedrich coauthored the experimental observation of X-ray diffraction by a crystal with Paul Knipping and Max von Laue. A beam transmitted through the sample produced separated spots on a photographic plate. Their geometry connected the periodic structure of the crystal with constructive interference of the scattered radiation. The apparatus turned a proposed wave test into an observable pattern, providing a method for investigating matter on atomic length scales.^[ref]

A physicist.^[ref]

Max von Laue proposed using a crystal's periodic atomic arrangement to test the wave character of X-rays. Scattering from regularly spaced atoms can interfere constructively only in selected directions, producing a pattern tied to the lattice. The measurements by Walther Friedrich and Paul Knipping connected that idea with photographic evidence. The result simultaneously linked short-wavelength radiation with a wave description and made atomic crystal order accessible to diffraction measurements.^[ref]

A physicist.^[ref]

The Laue X-ray diffraction experiment of 1912 connects a periodic crystal structure to a pattern of diffracted X-rays. Its history links a proposed wave test to the measurements made by Walther Friedrich and Paul Knipping.

An experimental physicist who coauthored a practical reconstruction of the Fizeau experiment. Comparing counterpropagating beams in moving water makes the small motion-dependent phase shift observable through interference. The setup links historical questions about light propagation to a reproducible optical measurement.^[ref]

A physicist who coauthored an undergraduate implementation of the Fizeau experiment. Its moving-water interferometer connects a small displacement of interference fringes to the propagation of light in a moving medium, giving a concrete measurement behind the historical discussion of optical drag.^[ref]

An experimental atomic physicist who coauthored a laboratory reconstruction of the Fizeau experiment. Counterpropagating light traverses moving water, and a shift of the interference pattern measures the motion-dependent optical path. The work shows how a historically important test can be made accessible in an undergraduate laboratory.^[ref]

Augustin-Jean Fresnel developed wave descriptions of diffraction and polarization of an electromagnetic wave. Fresnel diffraction retains the curvature of the propagation wave phase, letting a wave account predict patterns at finite distances from an aperture. His polarization work helped establish the transverse character of light. In a modern example, orthogonal linear polarizations have no interference cross term in an ordinary total-intensity measurement; the field directions matter as well as the relative wave phase. He also connected optical theory with practical lighthouse illumination through the Fresnel lens.^[ref]

A physicist.^[ref]

Hippolyte Fizeau tested the effect of moving water on the propagation of light. Two beams traversed flowing water in opposite senses and were recombined so that a travel-time difference appeared as a shift of interference fringes. Reversing the flow supplied a controlled comparison. His 1851 report compared the observed shift with competing accounts of how motion of a transparent medium affects light, making a small propagation change measurable without directly timing the transit.^[ref]

A physicist.^[ref]

Karl Ferdinand Braun investigated how coupled electrical circuits transfer and build up oscillatory power for wireless telegraphy. His lecture describes coupling several circuits of the same frequency into the transmitter's excitation circuit. Making them start to discharge at nearly the same time was a substantive experimental difficulty; he discusses a timing requirement of order $10^{-7}\mathrm{s}$ and an arrangement called energy coupling. The example connects circuit geometry and timing with the useful oscillation delivered to an antenna.^[ref]

A physicist.^[ref]

Wireless telegraphy connects an oscillating electrical circuit to a distant receiver. Tuning and coupling determine how effectively the circuit transfers a signal.

Guglielmo Marconi developed wireless telegraphy by coordinating the transmitter, propagation and receiver as one experimental system. His lecture describes a coherer containing nickel and silver filings, connected to a tuned circuit and a relay. The relay operates a recorder and a tapper that resets the detector, allowing successive signals to be registered. Long and short wave trains encode telegraphic signs. Improving detector reliability and resonance of electrical circuits therefore mattered alongside producing radiation at the transmitter.^[ref]

A physicist.^[ref]

Wireless telegraphy links the dynamics of a transmitter, propagation and the response of a receiver. A successful detector reading depends on that complete chain, rather than on the transmitter alone.

Philipp Lenard investigated cathode rays by allowing them to leave a discharge tube through a thin window. Studying their effects beyond the original discharge made their penetration, absorption and ability to ionize other material experimentally accessible. These measurements helped separate the properties of the rays from the particular gas discharge that generated them. His lecture describes the apparatus and the comparison of different materials, connecting an invisible beam with measurable fluorescence and electrical effects.^[ref]

A physicist.^[ref]

A physicist.^[ref]

Wien's displacement law connects a thermal spectrum's peak position to temperature. Specifying whether the spectrum is expressed per unit wavelength or per unit frequency is essential to stating which peak is meant.

A physicist.^[ref]

Lippmann photography records colour through the depth structure of an exposed emulsion. Its account connects a standing wave to a reflected optical spectrum.

An experimental atomic physicist whose work concerns the Zeeman effect and the Paschen-Back effect. At sufficiently strong magnetic fields, magnetic coupling competes with the internal coupling of angular momenta, changing the pattern of spectral lines. His book with Alfred Landé joins this atomic interpretation to the practical apparatus and analysis needed to resolve the splittings.^[ref]

Friedrich Paschen investigated atomic spectra in strong magnetic fields. The Paschen-Back effect, associated with his work with Ernst Back, occurs when the external magnetic interaction competes with internal spin-orbit coupling. The weak-field description through total angular momentum then ceases to provide the simplest labels. Separate orbital angular momentum and spin projections become useful, changing the pattern of spectral lines. Comparing weak- and strong-field patterns tests the hierarchy of interactions within an atom.^[ref]

A physicist.^[ref]

Alfred Landé organized the magnetic response of coupled atomic angular momenta. The Landé g-factor relates a magnetic energy shift to the projection of total angular momentum, accounting for both orbital angular momentum and spin. For LS coupling, $g_J=1+[J(J+1)+S(S+1)-L(L+1)]/[2J(J+1)]$ when $J\ne 0$, neglecting the small correction to the electron spin g-factor. For $L=0$ and $J=S=1/2$, it gives $g_J=2$. Different atomic terms therefore split differently in a weak magnetic field.^[ref]

A physicist.^[ref]

An atomic physicist who coauthored a NIST reference on atomic spectroscopy with William C. Martin. Its treatment of transition probabilities distinguishes the probability of an atomic transition from the population of its initial energy level. This matters because the strength of an observed spectral line depends on both atomic properties and the conditions of the emitting sample.^[ref]

An atomic physicist who coauthored a NIST compendium of atomic spectroscopy with Wolfgang Wiese. It connects the labeling of energy levels with spectral lines, transition strengths and selection rules. These conventions allow a measured spectrum to be compared with tabulated atomic data without confusing a level designation, a wavelength and an intensity.^[ref]

A radiation physicist who coauthored NIST reference tables for X-ray and gamma ray interactions with John H. Hubbell. The mass energy-absorption data use his calculations to account for how much photon energy is retained after interaction processes. This supports radiation-energy deposition calculations alongside the separate mass attenuation coefficient used for beam transmission.^[ref]

A radiation physicist who coauthored NIST tables of mass attenuation coefficients and mass energy-absorption coefficients with Stephen M. Seltzer. The tables connect X-ray and gamma ray photon energy to interactions in elements and mixtures. They distinguish removal of photons from a beam from absorption of their energy, a difference needed when calculating shielding or deposited radiation energy.^[ref]

A particle physicist who wrote a historical analysis of the Stern-Gerlach experiment. It distinguishes the experiment's original proposal and interpretation from its later connection with electron spin, and discusses the different problem of performing an analogous measurement on free electrons. This helps prevent later quantum concepts from being read back into the original experimental argument.^[ref]