That rule about why straws look bent in water actually applies to heat and chemicals, too, even though they aren't waves.
March 26, 2026
Original Paper
The Spectral Domain Snell Law in Diffusion-Wave Fields
arXiv · 2603.24094
The Takeaway
Snell's Law is a fundamental pillar of optics and wave physics, but it was thought to be impossible for 'diffusive' processes like heat or spreading chemicals. This discovery reveals that these processes follow the same geometric rules of refraction, opening up new ways to 'steer' heat or drugs through a medium.
From the abstract
Snell law is traditionally regarded as a hallmark of phase-propagating phenomena such as optical, acoustic, elastic, electromagnetic, and quantum waves. In contrast, purely diffusive processes, such as Fourier heat conduction and chemical diffusion, are generally considered incapable of exhibiting refractive/reflective behavior. In this letter, we demonstrate that although diffusion waves including thermal diffusion, mass diffusion, Lindblad quantum diffusion, and electromagnetic diffusion do no