Since light was first studied in the sixth century B.C., arguments had raged about its essence. Some Ancient Greeks considered light to flow in particles, like the atoms that made up all matter. Others thought it to be shimmering waves called eidola. Particle or wave? The debate summoned all of light’s mysteries.
Thomas Young’s sketch of two-slit interference based on observations of water waves. Credit: Wikimedia Commons.
International Year of Light Blog 