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I understand that a higher frequency, not a higher intensity, will be able to remove an electron. However, what I don't understand is why? Increasing intensity and increasing frequency are both forms of increasing energy.
The energy of the photon must be greater than or equal to the energy to remove an electron. Light sources with long wavelengths have a lower frequency. The lower frequency does not have enough energy to remove an electron, even if it has high intensity. Intensity is also the amplitude of the wave. The amplitude does not matter because the wavelength (frequency) is the only factor that determines what amount of energy needed to remove an electron.
In order to remove an electron, there has to be a photon and electron interaction. You can have a high intensity, meaning more photons interacting with electrons, however, this does not guarantee that electrons will be emitted. Instead, the photon must have greater energy than the electron in order to remove it. Therefore, frequency needs to be reduced for the photon to have more energy than the electron. Once that frequency is reached, then increasing intensity will only increase the number of electrons ejected.
By increasing the intensity of light, the amount of photons are increased, not the amount of energy each individual photon contains. Because each photon interacts with one electron, the amount of photons does not cause electrons to be emitted.
Yes, intensity is very different from frequency. We care about the fact that a photon needs to be of a greater energy than the electron to remove the electron, which is determined by a higher frequency as high frequency corresponds to high energy. Intensity merely means how many photons interact with electrons, which doesn't matter in a 1 to 1 interaction between photon and electron.
Increasing the intensity is not a form of increasing energy. Increasing intensity only means more photons are present, which doesn't change the overall energy level. However, increasing the frequency increases the overall energy level.
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