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Why does changing the frequency of a wave affect the energy?

  1. Apr 24, 2016 #1
    I don't understand how changing the frequency of a wave effects the energy transferred to electrons following E=hf, but changing the intensity doesnt? Can someone explain it please, some sort of analogy or something??
    THANKS!!
     
  2. jcsd
  3. Apr 24, 2016 #2
    I mean this also links to 'understanding how the photoelectric effect provides evidence for the particle nature of electromagnetic radiation', so if someone could explain that point as well that would be great as I guess understanding one will help me understand the other.
    Thanks again
     
  4. Apr 24, 2016 #3

    blue_leaf77

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    For a continuous illumination, changing intensity (and keeping the beam diameter constant) is equivalent to changing the number of photons contained in the beam. When you use this beam to knock out some electrons out of some material, changing intensity should be accompanied by a changing number of liberated electrons.
     
  5. Apr 24, 2016 #4

    blue_leaf77

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    Therefore, increasing the intensity will only increase the number of liberated electrons with the individual electron's energy remains unchanged.
     
  6. Apr 24, 2016 #5
    Thanks for the quick reply, but I honestly cant get my head round what your trying to explain, currently doing AS physics so a more simplified analogy or something would be great?
    Thanks
     
  7. Apr 24, 2016 #6

    vela

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    I'm sure this is explained in your textbook in detail. What do you think is happening in the photoelectric effect?
     
  8. Apr 24, 2016 #7
    But i think OP is asking about the frequency of an individual photon and not the amount of photons which I'd also like to know.

    The last post in this other thread says intensity is a combination of #of photons and frequency.
    https://www.physicsforums.com/index.php?posts/2468490
    I thought intensity was like what current is to electrons.

    Edit: but it seems 1 photon would not have a frequency but the wave version would. According to that thread. But there are higher energy photons I'm sure. What gives them more energy than other photons. Or frequency if you want to talk waves.
     
    Last edited: Apr 24, 2016
  9. Apr 24, 2016 #8
    Higher the frequency of the wave, more is the energy carried by it. A wave with higher frequency would have more energy, and thus more energy will be transferred to the electrons.
     
  10. Apr 24, 2016 #9
    Lol so it's another question where we're not supposed to ask why? These waves are being created. Lets say by an electron moving energy levels. So jumping from a really high level to a really low level will create a high energy photon? Can one electron only make one photon per jump?

    Bonus question: Is the radiation created from nuclear things we talk about from the ejected electron?

    Is annihilating an electron positron pair into radiation just mean the electron is moving to the lowest possible energy state? Meaning the same process as an electron moving energy levels?

    I will be happy if you ignore my questions and just answer the OP.
     
    Last edited: Apr 24, 2016
  11. Apr 24, 2016 #10

    Merlin3189

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    At the risk of getting banned from PF for ever, I'll have a shot.
    For a start, E=hf gives the energy for a single photon. THAT changes with frequency, but intensity just changes the number of photons with that energy. Photons with high frequency each have high energy and photons with low frequency each have low energy.
    Lame analogy: dropping a brick 1m from a table to the floor. Changing the density and mass of the brick (my analogy for frequency) changes the energy of the brick. Dropping ten bricks gives 10x the energy, but it's still the same energy per brick.
    I can get the same amount of energy from a small number of heavy bricks or a larger number of light bricks: I can get more energy from a lot of light bricks than a few heavy bricks, but each individual heavy brick will always have more energy than an individual light brick.
    So for light, the same energy could be few high frequency photons, or many low frequency photons.

    Lame analogy for frequency affecting the energy of the photon? (And physicists please excuse me here as I have no idea why it really does!!)
    Waves in the sea rolling onto the shore have energy - big lumps of water are lifted up and down. Each wave coming in can use the energy to move a float up and down, turning a generator and converting the wave energy into electric energy. Each wave of a fixed height can lift the float once and give a certain amount of electric energy. Waves with a higher frequency will do this more times per minute and therefore generate more electric power than waves with a lower frequency.

    If that keeps you happy, good. Personally, I don't worry so much about it. But I do have to warn you that there's little in common between mechanical waves and electromagnetic waves, apart from the name and some maths. As far as I know there's nothing in sea waves nor sound waves that corresponds remotely to a photon.

    MullaTheMech, you ask a very interesting question (to which I also don't know the answer!
    At one time I would have thought this, but having come across 2 photon microscopy, where two infrared photons seem to excite an atom to emit a visible photon, I'm not so sure. It would appear that one electron can absorb two photons simultaneously, so I guess the reverse could be true.
    (I haven't had time to go back to the real sources on this and the WikiP article Two-Photon absorption is tagged as dubious, but if necessary I can get plenty of solid references on 2 photon microscopy.)
     
  12. Apr 26, 2016 #11
    The textbook explains the basics, for me personally to accept that something happens I have to know why else I just wont remember it!!
    But thanks for reply, I think the other guy explained it will
     
  13. Apr 26, 2016 #12
    Thanks so much this actually makes a lot of sense, I think I was looking at it as a wave too much and not so much a particle.
    Analogy helped understand it, thanks again!
     
  14. Apr 26, 2016 #13

    Merlin3189

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    After posting I realised I had not addressed the quantum issue. With bricks, if we throw more bricks of the same size we can do more damage. But with the photo electric effect, it doesn't matter how many photons you have, unless one photon is energetic enough, no electrons get knocked off. Once individual photons have enough energy, each one can knock off an electron, even though there may be very few photons.
    I can't think of anything in the macroscopic world that makes an analogy. Which is why the photoelectric effect made people think about quantum behaviour.
     
  15. Apr 29, 2016 #14
    They are probably right not letting people try to explain it yet. Learning from google got me into an ether theory... atleast I know about that theory now.
     
  16. Apr 29, 2016 #15

    vela

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    You're right to doubt your analogy. At the introductory physics level, ##E=hf## is just an empirical fact. It was one of the first indications of quantum behavior discovered. There's really no classical physics way to explain it. Going forward, you also want to keep in mind that quantum mechanics is the more general theory, so quantum mechanics should explain aspects of classical mechanics, not the other way around.

    Be careful about what you read on the internet. There's a lot of BS out there.

    It's not so much that people aren't allowed to explain stuff here, but the answer to the OP's question is covered in most intro physics books. PF isn't here so students don't have to read their books. Also, if this was a homework question, the point of the exercise is to have the student go through the work to come up with the explanation or analogy because doing so requires the student to figure what's going on, which helps the student learn the material.
     
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