We consider random reflections (according to the Lambertian distribution) of a light ray in a thin variable width (but almost circular) tube. As the width of the tube goes to zero, properly rescaled angular component of the light ray position converges in distribution to a diffusion whose parameters (diffusivity and drift) are given explicitly in terms of the tube width.
"Knudsen gas in flat tire." Ann. Appl. Probab. 29 (1) 217 - 263, February 2019. https://doi.org/10.1214/18-AAP1412