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Problem with photoelectric effect

  1. May 17, 2003 #1
    Hi,

    I'm have a problem with photoelectric effect
    it states that we have a wavelength of 2.08*10^-7 falls on the photosurface, a voltage of 1.40V is required to stop the emitted electrons from reaching the anode.
    What is the largest wavelength of light which will result in emission of electrons from this photosurface.?

    so we have the equation
    eV=hf-W
    W=h* critical frequency and
    K=hf-W
    and
    f=speed of light/wavelength.

    v=1.40
    f=(3*10^8)/(2.08*10^-7)

    And wouldn't the Kinetic energy be 0 for a wavelength of 2.08*10^-7
    So i don't know where to go from here!! I want to do it but I can't it is frustrating!!
    Any help is appreciation
     
    Last edited by a moderator: Feb 4, 2013
  2. jcsd
  3. May 17, 2003 #2
    Hi again,

    I think i got it

    we have
    K(kinetic)= eV
    eV=hf-W
    1.4 * (1.6*10-19) = h c/wavenlength -work
    we get work =
    7.7*10^-19 (About)
    Now when it is the largest value above this value no electron will be emitted therefore KE=0
    so we would have
    0= Hc/waveleng-W
    or
    hc/w=wavelength
    and we get 2.6*10^-7

    But my question what would be the minimum value of wavelength?? if it is possible to find
     
  4. May 17, 2003 #3
    Your answer to the question is right (I got 272 nm without rounding). As for a minimum wavelength - there isn't really a restriction on how short wavelength the light is aslong as it is shorter than the threshold (maximum) wavelength. It's just that a higher potential would be needed to stop emission.
     
    Last edited: May 17, 2003
  5. May 17, 2003 #4
    Hi,

    one more quick question, as long as the frequency is below the threshold electrons are emitted, and above the threshold, the electrons are not emitted? Am i correct
     
  6. May 17, 2003 #5
    Damn sorry I've probably just confused you now. I've editted my original to say wavelength instead of frequency (oops). Aslong as the photons are above the threshold frequency, or below the wavelength at this frequency will there be emission. Sorry I kept swapping wavelength and frequency and probably confused us both!

    Anyway to make it clear again - aslong as the photons are above the threshold frequency, or have a shorter wavelength than the wavelength at this frequency there will be electron emission. So there is really no minimum wavelength value. lol :wink:

    editted: about 10 times for spelling and grammar
     
    Last edited: May 17, 2003
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