Arthur Schawlow shared half the 1981 Nobel Prize in physics with Nicolaas Bloembergen "for their contribution to the development of laser spectroscopy.” The other half of the prize was won by Kai M. Siegbahn "for his contribution to the development of high-resolution electron spectroscopy." Arthur Schawlow worked on microwave spectroscopy with Charles Townes, superconductivity, and nuclear quadrupole resonance. Spectroscopy is the study of the interaction of matter with radiated energy (such as electromagnetic radiation). For example, hydrogen only absorbs and emits specific frequencies of visible light that are related to its allowed electron energy levels. While working with Townes, Schawlow began researching what materials might produce coherent visible light, to create a LASER in the visible portion of the electromagnetic spectrum. After the realization of the LASER, he used the device to investigate the spectral absorption and emissions of various materials.