How did older fields such as mechanics and optics become part of modern physics?

Older fields like mechanics and optics were foundational to the development of modern physics, though their roles shifted significantly during the scientific revolutions of the 20th century.

Mechanics

Initially, mechanics served as the primary framework for understanding the physical world, rooted in Newton’s laws of motion. By the 18th century, it was transformed into a branch of mathematics, utilizing sophisticated analytical methods to solve complex physical problems. However, the 20th-century revolution revealed that classical mechanical models could not explain phenomena at the atomic level or at high speeds. This led to the development of relativistic mechanics, which redefined space, time, and mass, and quantum mechanics, which moved away from deterministic mechanical models toward probabilistic measurements.12345

Optics

Optics evolved from the study of light rays and wave phenomena into a core component of electromagnetism after the synthesis of electric, magnetic, and optical phenomena. While classical optics remains essential for understanding lenses and interference, the field expanded into quantum optics to address the quantum interactions of light with matter. This modern branch is now critical for testing the foundations of quantum theory and developing technologies like lasers and quantum computing.678