This is known as a self-mixing interferometer (as is explained in the video) https://en.m.wikipedia.org/wiki/Self-mixing_interferometry https://en.m.wikipedia.

This is known as a self-mixing interferometer (as is explained in the video) https://en.m.wikipedia.org/wiki/Self-mixing_interferometry https://en.m.wikipedia.org/wiki/Self-mixing_interferometry