A level Physics Quantum Phenomena

Hi TSR,

I am just a bit confused on this concept and if you could help me understand, it would be much appreciated.

If photons are bombarded at a metal, photoelectrons will be released. However, only if the photons are above the threshold frequency and can vary.

But if photons are absorbed by an electron in an atom, the electron moves to an outer shell. However, only a specific photon energy can move it to a different energy level.

So, how come different things happen when photons are bombarded at a metal and at an atom? Could someone just explain the whole concept to me please because I don't understand it.

Thank you so much for your help.

There are two ways that we can explain the differences.

For photoelectric effect, the photon energy is usually in the order of ultra-violet region while the photon energy that is responsible for the transition of electron in the energy level in the atom is usually in the order of visible light region.

The "fundamental" reason is usually not taught in A level unless you are taught energy band in a solid.
In an atom, the energy levels are discrete.
In a piece of solid metal, the energy level of the atoms forms a discrete and continuous band which sometimes it is called the band structure of the solid.
The atoms near the surface of the metal also form band of energy. Sometimes, people would refer them as surface electronic structure or surface band structure.

https://www.youtube.com/watch?v=yyofIfippOU
https://www.youtube.com/watch?v=xwAH7fQYASE
http://www.pci.tu-bs.de/aggericke/PC4e/Kap_II/Metalle.htm

Regarding your first point:

Don't you need to be above the threshold frequency for something to happen? If visible light is below the threshold frequency, surely nothing should happen?

Thanks for your help!!

An electron can absorb a photon and jump to different energy levels, where these energy levels are analogous to orbits around the nucleus (which I’m sure you know). The electron moves to different energy levels depending on the energy of the photon (defined by E = h•f). If sufficient energy is given to the electron, it can then become free from the atom. The photoelectric effect comes into play and is the net effect of the ionisation of the metal due to electron emission, as a result of the incident photons exceeding the work function of the metal. Both are, at the heart of it, are the same thing. Where the photoelectric effect seems to be the overall effect of electron emission from atoms. If the incident photons don’t result in electron emission from a metal, the electrons in the metal still will move around energy levels, but may not necessarily release photons.I hope this helped, I’m not the best at verbalising things😊Please someone correct me if I’m wrong.

A word of advice for A level physics students, avoid connecting two seemingly similar processes in physics if you intend to study physics in the university, so that you don't have to unlearn what you had connected in your A level study. Unlearning what was wrong is a painful and frustrating process.

I would advise against connecting ionisation of atom and photoelectric effect. The two processes may have "similar" effect of producing an electron but the energy spectrum of the emitted electrons for the two processes are different.

The energy level in an atom is discrete. When enormous number of atoms come together to form a solid material like metal, the energy levels of the electrons forms what is known as energy band. The energy level within the band are not really discrete. It may not make sense to say the electron that is emitted belong to a particular atom.

If you look up a solid state physics text, you can find the definition of work function relating to fermi level and vacuum level.
"The energy difference between the vacuum level and the Fermi level is defined as the work function (the energy cost of removing electrons from the system)"
OR
"The work function W of the uniform surface of a metal is defined as the difference in potential energy of an electron between the vacuum level and the Fermi level."

If you don't understand what I are writing, it is ok because most of them you will only learn in the university study in physics rather than at A level.

Perfect, thank you, that makes sense. I do hope to study physics this coming year (depending how Thursday goes), thank you for disconnecting the two for me! I assumed the two were one in the same thing, I guess assuming does make an ass out of me!