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Homework Help: The Photoelectric effect and classical physics

  1. Jan 3, 2007 #1
    The Photoelectric effect and "classical physics"

    The kinetic energy of the ejected electrons predicted by the classical physics should be related to the intensity of the light.
    According to experimental results, the kinetic energy of the electrons is proportional to the frequencies of the light, and not the intensity.

    What is "classical physics" in this case, and which equations do I use in order to calculate this?
    Why is the kinetic energy of the electrons depended on the intensity (according to "classical physics"), and not the frequency?
  2. jcsd
  3. Jan 3, 2007 #2


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    Look at the energy of a typical classical wave. What characteristic of that wave do you use to calculate its energy?

    Still confused? Look at a mass-spring system. How would the energy change if you change the amplitude of oscillation?

  4. Jan 3, 2007 #3
    Hmm, I don't know anyting about it. Could you list a couple of words, then I'll try to look them up or google them
  5. Jan 3, 2007 #4
    Simple Harmonic Motion. You will find all the information you need pertaining to energy, intensity, amplitude, power, etc.
  6. Jan 3, 2007 #5
    Classical physics refers more to the concept of light the Young proposed--that it is a wave and can be thought of much like ripples coming from water (the ripples being the spread of light). Einstein however explained that light should also be thought of as a bunch of particles. So in the ripple (classical) approach, if the ripples are bigger (that is, the light is brighter) then it should hit the metal harder and therefore electrons should eject with more energy. Because experiments disproved this, Einstein came up with the theory that light is a bunch of particles that also have wave properties which are called photons. Now the energy of each individual photon is proportional to its frequency. That explains half of it. The reason that brighter light does not eject electrons is because all that is is more photons (not stronger ones) . Think of shooting a billion ping pong balls at the Great Wall of China--not gonna do too much.
  7. Jan 3, 2007 #6


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    What exactly don't you know? You have learned about classical waves, haven't you? Figure out how energy in a wave is determined.

  8. Jan 4, 2007 #7
    Very good explanation. I'll dig into the physics stuff now that turdferguson and ZapperZ gave me some words to look up.

    Actually... no. I'm still in high school (or rather, the European equivalent) and this is my first year with physics. We're learning about waves at the moment, but this is _very_ fundamental (and probably simplified to the umpteenth power). I'm doing a assignment on the wave/particle duality (still on a very low level).
    But there is plenty to write about.
  9. Jan 6, 2007 #8
    Why can't waves have discrete values (E = hf applies to particles?)?
  10. Jan 6, 2007 #9


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    What makes you think that the energy distribution of photons is discrete?
  11. Jan 6, 2007 #10
    I figured it out.

    Thanks, everybody!
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