Endoscopy, a technique used to explore the stomach or other inner parts of the body is based on the phenomenon of

Endoscopes transmit illumination and images through bundles of glass or plastic optical fibres that guide light by repeated internal reflection at the core–cladding boundary. This guiding mechanism depends on total internal reflection: light entering the fibre strikes the internal surface at angles larger than the critical angle and is completely reflected, allowing rays to travel even around bends in the fibre. Modern fibre endoscopes use cores and lower-index claddings so light remains trapped and image-bearing bundles relay the view from inside the body to the observer or camera [1]. Thus the operating principle of endoscopy is total internal reflection.

Sources

1. [1] https://www.sciencedirect.com/topics/immunology-and-microbiology/fiber-optics