# What exactly is a threshold frequency?

For example, when you have a photon interacting with a bound electron, there is a work function relating the minimum energy photon capable of interaction (given by hf). My question is, what exactly is going on in this situation if the work function is exactly equivalent or less than the energy of the incident photon? How exactly does the photon and electron interact?

Also, is there anything similar to this? For example, an electron hitting a proton must have a particular energy requirement for interaction (right?). What exactly determines the threshold frequency necessary for interaction with a proton (is it the nuclear forces)? How about for structures composed of more than one atomic entity (e.g. sugar molecule)? Can a photon interact with the molecule as a whole or must it interact with distinct components (e.g. electrons, protons)? Can essentially anything interact with something else if it has enough energy?

I guess my question boils down to: do all things exhibit a "threshold frequency"?

Please let me know if you need any further clarifications or if I'm grossly mistaken on any points.

mfb
Mentor
How exactly does the photon and electron interact?
Physics cannot answer "how" questions on a fundamental level. We just observe that interactions do happen, and find equations to describe the result of those interactions.
For example, an electron hitting a proton must have a particular energy requirement for interaction (right?).
For inelastic interactions, where the particle contents change, yes.
What exactly determines the threshold frequency necessary for interaction with a proton (is it the nuclear forces)?
Energy conservation. The energy of hadrons like the proton is mainly determined by the strong force (and quark masses). For molecules, you also have to consider the electromagnetic interaction and the electron mass.
Can a photon interact with the molecule as a whole or must it interact with distinct components (e.g. electrons, protons)?
Both can happen, the energy of the photon is important.
Can essentially anything interact with something else if it has enough energy?
Yes.
I guess my question boils down to: do all things exhibit a "threshold frequency"?
Only if you want to create new particles, or reach excited states, or need the energy for something similar.

• MathewsMD
Physics cannot answer "how" questions on a fundamental level. We just observe that interactions do happen, and find equations to describe the result of those interactions.

Sure. Are there any particular models for this particular type of interaction?

Both can happen, the energy of the photon is important. Yes.

Would you possibly know of an example besides electrons and photons (I might be missing something here)? Are there any models by chance for this?

Thank you for all the help!

sophiecentaur
Gold Member
Would you possibly know of an example besides electrons and photons
Molecules of many gases have rotational and vibrational energy states and they will interact with appropriate EM frequencies ( Infra red and Microwave photons). RF waves will interact with vast numbers of conduction electrons in a metal antenna to cause currents - which can be detected with a Radio Receiver. So it's best to refer to interaction with 'charge systems', rather than just with 'electrons', as in the Hydrogen atom.

mfb
Mentor
Sure. Are there any particular models for this particular type of interaction?
Quantum electrodynamics.
Would you possibly know of an example besides electrons and photons (I might be missing something here)? Are there any models by chance for this?
An example of what? The question I replied to was specific to the interactions of photons with matter.

For non-photon-related interactions, you also get quantum chromodynamics (for the strong interaction) and the electroweak interaction (which unifies the electromagnetic interaction with the weak interaction). Both together form the so-called Standard Model of particle physics, which (at least in theory) allows to predict all interactions (as long as gravity is irrelevant).