Making a dark human hair transparent, or even an opaque bar of silicon: this optical "sorcery" is possible by manipulating the incident light. This new phenomenon is called "mutual extinction and transparency." Until now only existing in theory, photonics researchers of the University of Twente's MESA+ Institute demonstrated the effect with experiments. Applications include, for example, broadening the applicability of microscopy, as to include observation of opaque objects. The research team published its findings in Physical Review A.
source https://phys.org/news/2021-10-tuning-transparency-opacity.html
