There are several integration problems of fundamental physics that still lack coherent solutions, the case in point being wave-particle duality. While empiricism and mathematical modelling have served physics well, they have not yet been able to achieve integrated causal models. Conventional theories and approaches have only provided partial solutions, and it is possible that a more radical reconceptualisation of fundamental physics may be required. This work comes at the issue from a totally different approach: it applies design thinking to the problem. The result is the cordus conjecture, which proposes that the photon, and indeed every matter ‘particle’, has an internal structure comprising a 'cordus': two reactive ends that each behave like a particle, with a fibril joining them. The reactive ends are proposed to be a small finite distance apart, and energised [typically in turn] at a frequency. When energised they emit a transient force pulse along a line called a hyperfine fibril [hyff], and this makes up the field. This concept is used to explain the path dilemmas of the single photon in the double-slit device, and the wave behaviour of light including the formation of fringes by single photons and beams of light. In addition it provides a tangible explanation for frequency. It also yields new quantitative derivations for several basic optical effects: critical angle, Snell’s law, and Brewster’s angle. Thus the cordus structure offers an alternative conceptual explanation for wave-particle duality.