Energy Per Photon

[Ava Nickman]

I am still bit confused on the idea of energy per photon in the photoelectric effect. In an earlier lecture Lavelle was saying how the intensity of the light is not what ejects electrons but rather the frequency of the light. He said that increasing intensity of the light increases the number of photons but the energy per photon stays the same and that energy is lower than the threshold energy required to emit an electron. This made a lot of sense because h is a constant in the E=hv equation. However, in a later slide he says that if the energy per photon us greater than or equal to the energy needed to remove an electron, increasing the intensity of the light would result in electrons being emitted. Why is the photon energy not set? Why do we use a constant if it can change? What determines the photon energy?

[Chem_Mod]

Each photon must have the requisite energy required to eject a electron because one electron can only interact with one photon. Two photons cannot combine their energies and emit an electron if both have a lower energy than required. Photon energy is not constant, and that is seen in the Einstein equation: E = hv. h is a constant, but frequency (and wavelength) can vary, giving rise to many different photon energies. Experimentally, you would change the photon energy by using a different light source (e.g. UV light vs visible light).

[Kailani_Dial_2K]

Hi, so lets start with the first part of your post. So if you have a light that when it hits a metal, it does not eject an electron, then increasing the intensity of that light will not make an electron be ejected because the energy per photon is not enough to get over the threshold barrier. The reason that this is true in this experiment is because light is acting like a particle and not a wave. IF light was acting like a wave then increasing the intensity would increase the energy of the photons in the light thus resulting in an ejected electron. However, since this was not the case we know that light in this case is not acting like a wave it is a particle. Now that we have in mind that light is acting like a particle, let's talk about energy of the photon. The energy of the photon is determined by the frequency of the light. So in your statement where you talked about the energy staying the same because of Planck's constant is an incorrect approach. The reason that the energy of the photons do not increase when the intensity of the light increases is because the light is not acting like a wave. As I said before IF it was then increasing the intensity would have to increase the energy BUT it didn't, so we know that light is a particle. Lavelle states that increasing the intensity of the light when light is acting as a particle simply increases the number of photons. The photoelectric effect is a one particle one electron reaction. This means that one photon of light hits one electron and ejects that electron. So if you increase the number of Photons you can increase the number of electrons that get interacted with which means you increase the number of electrons ejected. However, this statement only applies if you were able to get electrons ejected with the frequency of light that you were using to begin with. The frequency of light is what determines the energy of the photon


Energy of the photon= h* frequency of light

I hope this helps