1801 : Thomas Young’s Wave Theory of Light

Young broke ground with an experiment that would become a staple in studying and separating waves and small particles: the double-slit experiment. Synthesizing Newtonian and Cartesian ideas on the subject, he theorized light was wave-like but transmitted perpendicular to its medium. This counterintuitive theory explained some phenomena, such as polarization, and introduced the idea of light’s medium: the ether.