33_photoelectric - Lesson 33: Photoelectric Effect Hertz...

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Lesson 33: Photoelectric Effect Hertz Experiment Heinrich Hertz was doing experiments in 1887 to test some of Maxwell's theories of EMR. One of the experiments involved using a coil of wire as a receiver to detect EMR produced by a separate device. This was very much like Maxwell's own experiments with low frequency AC radiation. Hertz was having problems seeing the spark that was made by the coil, so he placed the whole thing inside a dark box and looked through a glass window. Funny thing was, the sparks seemed shorter now. He could see them in the dark, but they just weren't as big anymore. Hertz noticed that when he changed the glass window to one made out of quartz, the sparks got bigger. The big difference is that the glass blocks UV, but the quartz does not. In the end, Hertz concluded that if random UV radiation in the room was able to go into the box (it could only do that if glass did not block it) it would hit the coil and help electrons pop off making bigger sparks. Hertz did not try to figure out why this was happening. Einstein's Explanation Threshold Frequency Einstein figured out that it was the frequency of the light hitting the metal that was important. When the UV light hit the metal of the coil, it had enough energy to knock off electrons. This was happening because the individual photons of UV had enough energy according to the formula E = hf. If the metal is exposed to radiation with a frequency less than UV, nothing happened. Since the frequency of the light is so low, each photon does not have enough energy to knock off the electrons. This critical minimum frequency that is needed to start knocking off the electrons was named the threshold frequency . The special symbol used for it in formulas is f o . Work Function Einstein believed that to give a single electron the energy to move, the metal was hit by a single photon (destroying itself), and transferred its energy to the one electron. Since the electron is originally attached to the metal, some minimum amount of energy must be
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33_photoelectric - Lesson 33: Photoelectric Effect Hertz...

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