Now, since the time of Newton there had been a debate about whether light was a wave---that is, a traveling disturbance in some background medium---o… - Lawrence M. Krauss

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Now, since the time of Newton there had been a debate about whether light was a wave---that is, a traveling disturbance in some background medium---or a particle, which travels regardless of the presence of a background medium. The observation of Maxwell that electromagnetic waves must exist and that their speed was identical to that of light ended the debate: light was an electromagnetic wave.