In part three of our biography of Johannes Kepler we look at his years in Prague and the scientific work he did there including Astronomiae Pars Optica, Dioptrice and Astronomia Nova.  We discuss how he arrived at his first two laws of planetary motion and his description of how lenses produce images in various optical systems including the eye.  We also follow the personal tragedy of the death of his son Frederick, how wife Barbara and the chaos that eventually engulfed Prague.