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How was the fact that long wavelengths, regardless of intensity, could not eject an e-, but shorter wavelengths could used to determine that light can be quantized as photons? What is the reasoning behind this conclusion?
For a metal to eject electrons, the ENERGY of the incoming radiation must match or exceeds the required energy to eject the election. Because E=hv and v is inversely proportional to wavelength (c=wavelength x v), that means that the wavelength of the incoming radiation, rather than the intensity, determines whether or not the electrons are ejected. The only thing that intensity impacts is the number of photons in the incoming radiation, which is reflected in the number of the ejected electron.
In the photoelectric effect, light with a long wavelength (and high intensity) in turn has a low frequency and therefore low energy. Because a photon shown onto a metal only has the capability of removing one electron, it will not matter how many photons there are if the energy of the light does not equal or exceed the threshold energy. If the energy of the light shown onto the metal is able to remove an electron, and there is a high intensity of photons, then an equal amount of electrons will be removed (as 1 photon correlates to 1 electron when energy is equal to or exceeding the threshold value).
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