Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B What is a quanta?

  1. Jun 18, 2016 #1
    I am very confused. Here are a couple of facts I cant connect :
    Quanta is the Delta of values of energy.
    Quanta is a packet of energy.
    a packet doesn't really stop at a finite distance from its peak.
    The Energy of a wave is the total energy of its quantas.
     
  2. jcsd
  3. Jun 18, 2016 #2
    After some more research I have more specific questions :
    what is the relationship between a wave and a photon? is every photon a peak? is every peak the sum of a couple photons which each represents a quanta?
     
  4. Jun 18, 2016 #3

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    You will not be able to make a classical analogue of what photons are. Electromagnetism is essentially as difficult as it gets when it comes to quantisation. A classical electromagnetic wave is not consisting of any fixed number of photons. The classical limit is provided by so-called coherent states, which is a superposition of states with different numbers of photons.

    The main message is that photons are not "little balls of light".
     
  5. Jun 18, 2016 #4
    That I understand, and because I know this it confuses me more.
    Ill be more organized :
    Is the energy of the wave is its intensity(which means total value over a specific time if I understand right)?
    are quantas levels of energy or the difference between the possible levels of energy? _folllow-up(if first one is true)_: is the total energy of the wave the sum of the different quanta?
    in many explanations people talk of a wave having a quanta multiple of photons, how photons relate to this?
     
  6. Jun 18, 2016 #5

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Energy and intensity are different things.
    What exactly do you call "quanta"? While "is quantized" is used frequently, I rarely see that word as subject. Do you mean photons? The energy of photons emitted by an atom (for example) corresponds to the energy difference between two levels of that atom.
    For many waves, that sum is not even well-defined.
     
  7. Jun 18, 2016 #6
    firstly, I would like to mention my appreciation for both of your comments.
    Now lets get to my questions :
    If the energy of a wave is not its intensity, than what is it? and how do you convert it to other stuff?
    some sources I read used photons as examples to quanta. Some said its the difference of wave levels. And some said the Energy of a wave is the sum of its quanta's energy. Now its really starting to sound like photons.
    The main things that confuse me are the quanta/photon-wave relation(it sounds like photons are parts of waves instead of having waves describing them which is what I thought in the beginning) and what the wave function describes.
    does it all relate to superposition and having many frequencies at the same time?

    Sorry for being so ignorant. I am 15 and stupidly volunteered to do a presentation on this topic.
     
  8. Jun 18, 2016 #7

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Intensity is power per area. Asking if energy is "inside it" does not make sense. It is a different physical quantity.
    Forget about waves if you want to consider photons. That doesn't work well together because it is very unintuitive.
    Superposition is often relevant, different frequencies not necessarily.
     
  9. Jun 18, 2016 #8

    Drakkith

    User Avatar

    Staff: Mentor

    A greatly simplified explanation is that a photon is a quanta of energy associated with an electromagnetic wave. This just means that when an EM wave interacts with matter it does not do so in a "smooth" manner. As the matter absorbs the energy of the EM wave it does so in "chunks" or "packets", with each packet of energy being deposited all at once. We call these packets "photons". This is important because prior to the development of Quantum Physics, all waves were thought to deposit energy in a smooth, continuous manner.

    It also turns out that the energy level of electrons within atoms is also quantized. This means that the energy levels cannot smoothly vary from one value to another. Instead they can only be certain amounts, corresponding to certain orbitals in a manner that's much too complicated to explain in detail. Imagine if a spacecraft orbiting the Earth couldn't choose any arbitrary orbital height, but had to be in one of only a few possibilities. Perhaps they could only choose ones that were 1.1x the height of the last orbit, or ones whose height in miles were multiples of 100. These orbits would be quantized.
     
    Last edited: Jun 20, 2016
  10. Jun 20, 2016 #9
    I think I am starting to understand this.

    What I understand is that the waves describe the path of the photon to the interaction which happens all at once becouse its a quanta. The energy of the photon is related to its frequency. Are different interactions/photons derived from the same wave or does every atom has its own wave?
     
  11. Jun 20, 2016 #10

    vanhees71

    User Avatar
    Science Advisor
    2016 Award

    Forget photons at your stage of your studies. They are by far the most complicated case you can think of, because they are massless and thus do not even have a position observable in the strict sense.

    Start with non-relativistic quantum theory. A nice introduction is given in Suskind's "Theoretical Minimum".
     
  12. Jun 20, 2016 #11

    Drakkith

    User Avatar

    Staff: Mentor

    I agree with the others that you should probably not think about photons at this stage in your education, but that obviously doesn't help you if you're doing a report on the subject. If you're going to talk about photons, just keep it simple. Paraphrase my earlier post if you need to. Don't worry about the details of how photons relate to EM waves other than the fact that they are quantized energy packets.

    We seem to have covered photons and electron energy levels in an atom so far. What are some other examples of quantization that would help the OP in their presentation?
     
  13. Jun 20, 2016 #12
    I think an explanation of how qunatization of electrons and photons are connected can help. As the first one is levels of energy and the second one is the absorbtion of energy. I see that they both have quantization - they "jump" from one possible value to another; but is this the only connection or is there something I am missing? If so, its a bit blurred. And how do electrons act to photons in frequencies that dont allow them to jump? Do the photons "bounce" instantaneously or is there a chain of reactions happening?

    If I am right you said not to think about the wave-partivle duality so the questions above arent supposed to be as complicated.
     
  14. Jun 20, 2016 #13

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    The energy levels are not properties of the electrons, they are properties of the atom, molecule, or the whole material, depending on what you consider.
    Not at all. Examples are air and glass for visible wavelengths: the light just passes through.
     
  15. Jun 20, 2016 #14

    Drakkith

    User Avatar

    Staff: Mentor

    The only connection is that it takes energy to make an electron jump to another energy state and a photon is one method of providing that energy. A collision between two atoms or molecules is another method.
     
  16. Jun 21, 2016 #15

    vanhees71

    User Avatar
    Science Advisor
    2016 Award

    Ok, if you are forced to report on photons, then you should do it right and start with the quantization of the electromagnetic field. The most simple way is to give up manifest Lorentz invariance and work in the radiation gauge of the free electromagnetic field and then quantize the theory in terms of the two physical degrees of freedom (e.g., in the momentum helicity basis of single-photon modes).

    This approach is very nicely explained in Landau&Lifshitz, vol. IV.
     
  17. Jun 21, 2016 #16

    Drakkith

    User Avatar

    Staff: Mentor

    I think that may a bit too advanced for the OP.
     
  18. Jun 21, 2016 #17

    vanhees71

    User Avatar
    Science Advisor
    2016 Award

    It's not too advanced. It's the minimum level of sophistication to start defining what a photon actually is. One must simplify things as much as possible but not more (free quote by A. Einstein).
     
  19. Jun 21, 2016 #18

    Drakkith

    User Avatar

    Staff: Mentor

    Based off of what the OP has already written, and the fact that this thread is marked "B", I'm going to disagree.
     
  20. Jun 21, 2016 #19

    vanhees71

    User Avatar
    Science Advisor
    2016 Award

    Then don't even think about photons! Period. Start with non-relativistic QT and study the harmonic oscillator thoroughly. That's a much better preparation for tackle the photon issue than learning "old-fashioned quantum theory", which imho shoud be banned from the physics curriculum at any level anyway. It has it's place in a history-of-physics lecture but not in a physics lecture.
     
  21. Jun 21, 2016 #20

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    The OP is 15 years old.
    Any presentation on that level is going to have to be essentially popular science with all its merits and flaws.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: What is a quanta?
  1. Plancks quanta? (Replies: 3)

Loading...