How did Isaac Newton’s theory of colors differ from earlier ideas about white light?

Before Isaac Newton, the prevailing scientific view—dating back to Aristotle and later supported by René Descartes—held that white light was simple and homogeneous. This "modification theory" suggested that colors were merely the result of white light being altered or modified by external phenomena, such as passing through a prism or forming a rainbow.1

Newton fundamentally challenged this by demonstrating that white light is actually complex and heterogeneous. Through his experiments with a spectrum projected in a darkened chamber, he proved that white light is a mixture of simple, immutable components. He concluded that colors arise from the analysis—or separation—of this heterogeneous mixture into its constituent parts, each of which refracts at a distinct angle. This discovery of the heterogeneity of light replaced the modification theory and became the foundation of modern physical optics.1