1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Photoelectric effect and Compton scattering

  1. Mar 25, 2015 #1
    1. The problem statement, all variables and given/known data
    A photon with of 13600eV energy interacts with a hydrogen atom at rest and ejects the electron (photoelectrically) in the direction in which the photon was travelling. If 13.6 eV is required to eject the electron, find the speed of the photoelectron and the momentum and energy of the recoiling photon.

    2. Relevant equations
    Energy of a Photo: E = h*f

    Photo electric effect: KEmax = hf - phi

    Momentum for a photon: p = h / lamda

    Compton Effect: lamda ' - lamda = h / Me*c (1 - Cos theta)

    3. The attempt at a solution
    Now I'm confused. In class we learned about the photoelectric effect and the Compton Scattering separately. I understand and can do problems about both concepts. What I don't understand here is the ejection of the electron.

    Is the electron ejected due to the photoelectric effect or due to a collision with the photon?

    I know that Phi is 13.6eV. This energy will be lost due to pulling the electron out of the atom.

    Is the Kinetic energy of the ejected electron coming from the collision with the photon.


    Thanks for the help!
     
  2. jcsd
  3. Mar 26, 2015 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    ... it is not useful to think of the interaction as a "collision".
    It is just an interaction ... electrons and photons are not like little balls that have surfaces that can strike each other.

    ... the problem statement specifies "photoelectrically", but also says there is a recoil photon. In the photoelectric effect the incoming photon is completely absorbed by the electron so there is no "recoil photon" so I see your confusion.
    http://hyperphysics.phy-astr.gsu.edu/hbase/mod2.html

    I think you need to get a clarification from the person who wrote the problem.

    In the absence of a clarification, I'd combine the information ... treat it as a conservation of energy and momentum problem without worrying about what name to use.
    You have some initial state, something happens, and you get some final state. You need to relate energy to momentum and the initial state to the final state.
     
  4. Mar 26, 2015 #3
    Thanks for the reply! Sadly this question is for an assignment I have to hand in tomorrow and I won't be able to get clarification on the problem.

    I understand that the photon is absorbed. The question just confused me. The projected electron takes the energy of the incoming photon minus the work function of 13.6 eV. This will give it its Kinetic Energy. I can therefore find it's velocity, momentum and total energy.
     
  5. Mar 26, 2015 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    In the absence of a clarification, I'd combine the information ... treat it as a conservation of energy and momentum problem without worrying about what name to use.
    You have some initial state, something happens, and you get some final state. You need to relate energy to momentum and the initial state to the final state.

    ... you could take it like that, but then: how will you answer the second part of the problem - the bit about the recoil photon?
     
  6. Mar 26, 2015 #5
    I just read the section on the photoelectric effect in my book and it says clearly that the photon transfers all its energy to the electron. Therefore there is no photon to speak of...
     
  7. Mar 26, 2015 #6

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    So your answer to the second question is that there is no recoil photon?
    Fair enough - let me know how it went.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted