Atoms have electrons orbiting around a nucleus, and when an electron falls into an inner orbit it releases extra energy as an X-ray. Barkla verified this experimentally and showed that the wavelength of the emitted X-ray is fixed for each element. This is called a “characteristic X-ray.” His work paved the way for Henry Moseley, who connected X-ray wavelength with atomic number, helping reorder the periodic table and predict new elements. Barkla’s findings also evolved into non-destructive inspection methods now used in manufacturing, medicine, and forensic science.

Barkla irradiated target metals with primary X-rays and analyzed the secondary X-ray spectrum, detecting distinct K and L series lines. To demonstrate that X-rays are electromagnetic waves, he measured polarization using malachite powder rather than crystal diffraction, confirming the presence of linear polarization. These results matched models of intra-atomic electron transitions and served as critical empirical data preceding the Bohr model and quantum mechanics. Characteristic X-rays form the theoretical basis of modern X-ray fluorescence (XRF) analysis, widely employed in materials science, geology, and cultural heritage preservation.

Barkla distinguished edge structures from the continuous spectrum in the λ≈0.05–0.15 nm region and, through angular studies of scattered and fluorescent radiation, measured a polarization degree of P≈0.6. He quantified the strong Z-dependence of secondary line intensity, implying inner-shell vacancy filling after ionization. Preceding subsequent multiple-scattering theories by Shaw and Satyananda and early Brillouin analyses, Barkla’s work provided decisive guidance for a microscopic picture of X-ray–matter interaction. Contemporary techniques such as resonant inelastic X-ray scattering (RIXS) and time-resolved XRF still adopt Barkla’s polarization measurements near absorption edges.