Instead of letting it pass, the holes would reflect the light back toward the tip of the fibre optic cable, effectively turning the diaphragm into a mirror even as it still allowed water to pass.

Diffraction: When light rays pass close to opaque surfaces, such as the blades of a lens diaphragm , they are scattered;

A diaphragm in the plug body ruptures to allow the cement slurry to pass through after the plug reaches the landing collar.