Though a German physician, Julius Mayer contributed towards ans. Optics Radioactivity Thermodynamics Atomic structure

Julius Mayer is best known for his contributions to the field of Thermodynamics. He was a pioneer in the formulation of the law of conservation of energy, particularly in relation to heat. In the mid-19th century, Mayer proposed that energy cannot be created or destroyed but can be transformed from one form to another. His ideas were instrumental in the development of the first law of thermodynamics, which is foundational for understanding energetic processes in physical systems. Mayer's work helped establish the principles that govern thermal energy, leading to significant advancements in both physics and engineering. This laid the groundwork for numerous applications in various scientific fields, including mechanics, chemistry, and even biological systems.

In summary, the correct answer is Thermodynamics.