1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

De Borglie's wavelength equation

  1. Mar 1, 2013 #1
    1. The problem statement, all variables and given/known data

    A molecule is 3 nanometers wide and weighs 5 x 10^-15kg. It is fired through a slit that is 5 nanometers wide. Approximately how slow does the molecule have to go so that it diffracts?

    2. Relevant equations

    I'm thinking that I could use De Borglie's wavelength equation: λ=h/m=h/mv and then solve for v but since I don't know that lambda is I don't see how I can use it.

    Does anyone have a clue on how to solve this?
  2. jcsd
  3. Mar 1, 2013 #2
    Re: Diffraction

    What is "diffraction'?
  4. Mar 1, 2013 #3
    Re: Diffraction

    Quote from Wiki: "Diffraction refers to various phenomena which occur when a wave encounters an obstacle. In classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings"
  5. Mar 1, 2013 #4
    Re: Diffraction

    The question is, is there any relation between the size of the obstacle and some characteristic of the wave for diffraction to become significant?
  6. Mar 1, 2013 #5
    Re: Diffraction

    If I remember correctly, doesn't the wavelength have to be pretty small in order for the particle to diffract? So could I try different values for v and see what my λ turns out to be?

    I still don't understand what the slit width has to do with the question. Is it just there to show that the particle is able to go through it?
  7. Mar 1, 2013 #6
    Re: Diffraction

    You quoted Wiki: "spreading out of waves past small openings". How "small" must the opening be for spreading to become significant?
  8. Mar 1, 2013 #7
    Re: Diffraction

    Ahh, so the wavelength should be approximately the size of the slit right?
  9. Mar 1, 2013 #8
    Re: Diffraction

  10. Mar 1, 2013 #9
    Re: Diffraction

    Alright so since the width length 5nm, λ≈5nm or 5x10^-6m

    So v=h/(5*10^(-15)*5x10^-6m)≈2.65*10^-14m/s

    Is that reasonable? It seems like that is an extremely slow speed...
  11. Mar 1, 2013 #10
    Re: Diffraction

    Yes, it has to be slow, but not quite as slow. 1 nm is 10^-9 m, not 10^-6 m.
  12. Mar 1, 2013 #11
    Re: Diffraction

    Oh yeah, sorry about that. And thank you so much for the help, it was very informative.
  13. Mar 1, 2013 #12


    User Avatar
    Homework Helper

    Re: Diffraction

    Well, it was an extremely big molecule.

    Think: water molecule, for example has molar mass of M=0.018 kg. The mass of one molecule is M/A (A is the Avogadro number, 6x1023) So the mass of a water molecule is about 3x10-26 kg.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted