The Photoelectric Effect The exploration of interference phenomena had indicated that light existed as waves—in contrast to gases, for example, which were clearly composed of discrete particles. Right at the start of the twentieth century, however, experiments with cathode ray tubes had led to a phenomenon called the photoelectric effect that “classical” approaches to physics could not seem to explain. Certain metals like sodium and aluminum, when exposed to some kinds of light in a vacuum, triggered the release of a mysterious ray of particles—later identified as electrons—that would rush from a negatively-charged surface (the cathode ) toward a more positively charged one (the anode ). Two anomalies to this behavior were especially perplexing: 1. The speed of the rays was proportional to the frequency of the light, not the intensity . In fact, low frequencies of light would not even create the rays no matter how brightly it was shown on the metal cathode. If light waves were adding energy to the atoms in the metal, any kind of light at
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