In the 17th century, physicists such as Newton and Hooke debated whether light were a particle or a wave. Decisive experiments around the turn of the 18th century resolved these debates conclusively in favor of the classical wave theory. But in the early part of the twentieth century, experiments that studied the photoelectric effect required light to also have particulate properties. Theoretical explanations to resolve these disparate pieces of evidence gave birth to the modern quantum theory of light, where particles may act, in some sense, as both waves and particles simultaneously.
