Answer
The answer to the question lies in the realm of precision spectroscopy. Hänsch's Nobel Prize-recognized work centered on developing innovative methods for ultra-precise measurements of light frequencies, particularly in the visible and ultraviolet ranges of the electromagnetic spectrum. This extraordinary level of precision has significantly advanced our understanding of atomic and molecular structures and facilitated the development of various cutting-edge technologies, such as optical clocks, high-resolution spectroscopy, and precision metrology.