Photoelectric Effect

[Emma Li 2C]

For the photoelectric experiment, what is actually happening when light "knocks" electrons off of the metal?
I thought that it meant electrons are excited to a higher energy state by photons of a high frequency; however, after reading that electrons can only move to other energy levels with unique frequencies of light, I'm a little confused.

[Lena Nguyen 2H]

In the photoelectric experiment, the electron is completely removed or ejected from the metal. In this case, the energy the electron is hit with only has to be greater than the threshold energy and not an specific amount. This experiment wasn't about moving electrons to higher energy levels, it was about ejecting them from the metal.

The atomic spectra experiment is a different experiment. In this experiment, the electron is not completely removed by the light, and it will return to its ground state after it is excited. This experiment is where the energy levels are explained, and so the energy that hits these electrons have to be of a specific energy, which is the energy difference between the levels.

Hope this helped!

[Vivian Nguyen]

Hello!
Although my understanding is a bit rough, the photoelectric experiment is different from what you were describing. The photoelectric experiments exposed the particle model of light since electrons would not be ejected unless the waves had a short wavelength/high frequency. The photoelectric experiments provided ways to find the energy necessary to eject a bound electron from different types of metals - this is called the work function/threshold energy.

Meanwhile, there is another series of experiments involving spectroscopy and atomic spectra that covers the energy levels you were talking about. In this case we're talking about excited electrons not ejected electrons. Using the energy levels and corresponding unique frequencies of light, it tells us about the structure of the atom.

Hope this makes sense!

[Michelle Steinberg2J]

Hi -

Would someone be able to explain why a light's wavelength determines whether or not a metal can eject an electron? I have memorized that long wavelengths cannot and short wavelengths can, but I do not fully understand why.

Thanks!

[Chem_Mod]

In order to get knocked off of the metal, an electron has to be excited to n=infinity. When it gets here the electron is essentially free from the nucleus. Any extra energy that is left will give the electron its velocity.
Hope that helped!
-Ajith

[Morgan Baxter 1E]

You say that you have memorized " long wavelengths cannot and short wavelengths can" in terms of wavelengths of light that can eject electrons from metal. From my understanding, it is not that all short wavelenghts can, it is just that having a shorter wavelength makes it so the frequency of the wave is higher. This means that the energy per photon is also more. Every metal has a threshold energy that is required to have the electrons eject. The higher the energy per photon, the more likely it is that the energy per photon will be sufficient to overcome the threshold energy and it will be able to eject. While short wavelengths are more likely to be able to eject electrons, it is because short wavelengths are associated with higher energy per photon.

[aTirumalai-1I]

I think I understand the concept of the photoelectric effect, but I still don't completely understand why it can occur only in a vacuum. Can someone clarify that for me? Thanks!

[Swetha Sundaram 1E]

The photoelectric experiment has to be done under a vacuum because otherwise the ejected electrons interact with molecules in the air.

[kaitlindaugherty3L]

So in the atomic spectra experiment, electrons are never ejected? They only move to different levels, or is there a scenario where electrons can be ejected in the atomic spectra experiment?