During this period wave theory matured by merging diffraction into a geometric framework, enabling a unified treatment of how waves propagate, diffract, and interfere in practical systems such as antennas, radar, acoustics, and fluids. Researchers developed exact and approximate nonlinear wave descriptions for capillary and gravity waves, established envelope dynamics and modulation concepts, and advanced two-dimensional packet theories that balance group velocity with long-wavelength effects. The period emphasized cross-disciplinary methods, combining analytical solutions, stability analysis, and asymptotic techniques to reveal how small perturbations evolve into complex wave patterns.