Chapter 2 reviews answers to the question of what is light, starting with the ancient Greeks and ending in 1900 with the wave concept of Maxwell’s electrodynamics. For some ancient Greeks, light consisted of atoms emitted from surface of the object, whereas for others it was fire that either entered into or was emitted by eyes, although the latter possibility was effectively eliminated around the year 1000. Competing proposals well after then were that light is either a wave phenomenon or consists of particles, with Isaac Newton’s corpuscular (particle) theory prevailing by the end of the 1600s over the wave concept championed by Christiaan Huygens, who published the first estimate of the speed of light. In the early 1800s, Thomas Young’s two-slit experiment proved that light was a wave, a concept codified and firmly grounded through Maxwell’s theory of electromagnetic waves.
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