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Wavelength, energy and photons.

  1. Feb 20, 2012 #1
    This follows some questions/points made in my light and sight thread.
    I have been trying to work out the relationship/difference between photons and waves.
    I now think that a photon is a discrete particle like amount of energy needed to jump an electron from one orbit to another, perhaps call it the particle side of light. If this is the case then only certain wavelengths could be described as photons as once beyond a certain length they cannot have enough energy to react directly with an electron.
    I have been given E=hf for the energy of a photon where h = plancks constant and f= frequency. Frequency is obviously time dependant.
    If we start with a wavelength/frequency of 1 light second then we get E=h or energy transfer equal to h per second. Inverse law - wave 1/2 light second will transfer energy twice as fast giving appearance of twice energy + 2 waves per second = energy transfer 4x single wave.
    So is energy transfer via EM waves also time dependant, what I mean by this is that all waves carry the same energy but waves act like crumple zones causing that energy to be transfered over a longer period of time.
    Classical example take a rope with a breaking strain of 1 ton if you lift 1 ton slowly (long wave) the rope will lift the weight, if you jerk the rope (short wave) it will break.
    Another take a compressed spring (short wave) place one end against an object, say a ball, hit the other end virtually all the energy will be transfered immediately through the spring, now take an uncompressed spring (long wave) and do the same, less or the same energy(not sure how much energy the spring will absorb) will be transfered over a longer time, the longer the spring the greater the effect untill eventually the spring will be long enough that the energy transfer will be virtually zero same as EM waves. This also would explain the photoelectric effect, shorter waves cause electrons to be emitted faster not because there is more energy per wave but there is more energy per second being transfered.
    Has anyone tried using planck length for wavelength in E=hc/λ.
    How do long waves interact with matter as they dont have enough energy to react directly with electrons.
    Regarding electrons, energy makes them jump orbits, do energy levels for each orbit vary for different elements, i e is the energy for orbit 1 in hydrogen the same as orbit 1 for say gold?
    What is the maximum number of orbits allowed?
    Take hydrogen for example the electron presumably sits in orbit 1 normally, how many orbits can it be made to jump.
  2. jcsd
  3. Feb 21, 2012 #2
    You are probably not getting answers because you are making a lot of guesses which incorrectly describe known physical processes.

    While some light does not react with electrons, its is still light.

    Your question about E = hf mixes quanta and waves....E =hf applies to quanta not wave characteristics.

    Of course not....are you familiar with weak radio signals for example...as a radio signal fades?? Why do you think LASER
    weapons are being developed....because they can be made to carryenough energy to destroy things, even it 'space'.

    well, E = hf explains the energy transfer... but using a continuous model like a compressed spring will not adequately describe a discrete [photoelectric] quanta effect.
    Such energy transfer is NOT continuous.

    If all the energy from one photon is absorbed by one electron, some of the energy frees the electron from the binding energy of the atom, and the rest contributes to the electron's kinetic energy at release. Otherwise, the energy is re emitted and the electron remains bound.
  4. Feb 22, 2012 #3
    Thanks for the reply but I wonder if you have misread or misunderstood slightly.
    My guesses are based on all the formula I have found for energy.
    You say E=hf only applies to quanta yet it is the same as E=hc/w (w=wavelength) as f=c/w so wavelength must be related to quanta and quanta is pointlike rather than a wave.
    The Laser example does not prove I am wrong just that it is possible to transfer energy to an object faster than it can lose it.
    The compressed spring model is not a continuous model it is meant to represent a single wave and how energy is spread out along that wave and showing how the same amount of energy can be made to look small or large depending on how long it takes to transfer from one thing to another you have to imagine the compressed spring as very short wave or bunched up so that all energy is transfered in an instant rather than fed in as you would get with uncompressed spring energy is only completely transfered when spring reaches full compression you also have to think of the springs as moving forward not stationary. Its all about energy being transfered faster than it can be lost.
    Am still hoping for information regarding electrons and different atoms as mentioned at end of last post.
  5. Feb 22, 2012 #4
    That was a good start, but ending with a self contradiction. The logical conclusion from your argument is that a photon has a wavelength, which is not point-like. That has also been mentioned in a few other current threads, for example:

    https://www.physicsforums.com/showthread.php?t=576212 (see post #8)
    Last edited: Feb 22, 2012
  6. Feb 22, 2012 #5
    To avoid any confusion with the spring example each spring represents a single wave.
    Just guesssing but is a quanta the result of a collapsed wave?
  7. Feb 22, 2012 #6


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    A photon's position and momentum are described by the wavefunction. This is a basically a bunch of overlapping waves describing possible values of each property. The waves will constructively and destructively interfere with each other, resulting in a wavefunction that contains the probabilities of a particle being somewhere or having some value such as momentum. Because it describes probabilities, it is the wavefunction that must "collapse" from an infinite amount of waves into just one when it interacts.

    Now, this wavefunction doesn't tell you exactly where the photon is at, but rather tells you the probability of being somewhere. When you increase something's momentum the wavefunction's spread of possible spots for it to be becomes smaller, meaning that it is more likely to be found in smaller area and less likely to be found further from the center of the spread. Higher energy photons have more momentum and therefore smaller "spreads" for their position.

    No, wavelength itself has nothing to do with quanta other than to determine the energy a photon has, per e=hf.

    I'm sorry, this is incorrect. There have been many many experiments showing that light interacts in quanta. In the photoelectric effect electrons are ejected at the same interval that a photon is released. Furthermore, increasing the frequency of the light ejects the electrons at a higher velocity, but does NOT lead to more ejected electrons. On the other hand increasing the intensity of the light, which is the number of photons released per second, does increase the number of ejected electrons, but not their velocity. A classical wave would both eject more electrons and at a higher velocity if energy was transferred continuously, and it would do so if you just increased the intensity but not the frequency. This does not happen because photons only transfer energy in discrete amounts. And finally, the electrons have been observed to be ejected effectively instantly from the metal when the light is absorbed. This implies that energy transfer happens nearly instantaneously. Otherwise you'd have to explain how a wave, which transfers energy continously, isn't increasing the electrons kinetic energy to intermediate values, heating the metal.

    The energy can be transferred to the atom or molecule as a whole, increasing its energy. This typically manifests as simple thermal motion, aka heat.

    I don't believe each element's orbitals are the same energy. To my knowledge the more protons the atom has the less energy each orbital has as well. (AKA it takes more to remove an electron from an orbit in a lead atom as it does from the same orbit in a carbon atom.) But I'm not sure on this.
  8. Feb 23, 2012 #7
    Drakkith let me firstly say how good it is to find someone with the patience and knowledge to put up with me.
    I asked about the electrons in atoms because different elements show different (absorbtion?) lines in the spectrum which infers different energy levels. I always thought that all electrons were the same but if they absorb different wavelengths according to the element they are part of this raises questions.
    I have also been wondering about wavelengths beyond the radio frequencies there seems to be little or no information available.
    When you increase momentum you either increase speed or mass or both, waves move at constant speed so in order to increase momentum you have to increase mass as waves are supposedly massless how do you increase momentum? as you only have energy to work with the only way I can think of is speed of transfer, the faster energy is transfered the greater the force it appears to have.
    Another example buckets of water to represent energy. sessaw container at one end hammer and bell at the other, tip a bucket into container in one go the hammer will rise with enough force to ring the bell, tip it slowly the hammer will still rise but will only touch the bell without ringing it, admittedly you could say this is down to gravity, but in both cases you have used the same amount of energy.
    Back to the photoelectric effect, higher frequency = faster energy transfer so electrons would move faster, the faster you move your foot kicking a ball the faster it leaves it, increasing the intensity would be the same as using more feet kicking more balls at the same time.
    I am not talking about continuous energy transfer, e g a frequency of 1 million waves per second the energy would be transfered in 1 millionth of a second per wave, a wave with a frequency of 1 second would transfer the same energy in 1 second. Another question is can time be quantised.
    Thanks also for the info on wavefunction, is it the collapse of this via interaction/detection that explains the dual slit experiment?
    Somewhere there is an explaination of the natural constants.
    Do you have any knowledge about why some particles decay, eg whys a neutron stable in an atom and decays when on its own?
  9. Feb 23, 2012 #8


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    That's not because the electrons are different, but because their energy levels in their orbitals around the atom are different. http://en.wikipedia.org/wiki/Atomic_orbital

    Longer or shorter wavelength? Longer wavelengths typically begin to be described by electrostatics and magnetostatics, as the frequency becomes so low you can effectively treat it as a static field.

    Photons are massless, but they do have momentum. http://en.wikipedia.org/wiki/Photon#Physical_properties

    No, if the hammer touches the bell at all it will ring. The harder it hits the louder it is, but even the lightest touch will cause the bell to vibrate slightly. And this analogy simply doesn't work for light.

    Your not using your terms correctly. Frequency is not the number of waves per second, it is how fast the electric and magnetic fields of an EM wave oscillate back and forth. It is measured in Hertz. So a wave with a frequency of 1 hertz oscillates one time per second. Visible light is in the Terahertz range, which means it oscillates trillions of times per second.

    Have you read this article yet? It explains much of what you are asking. http://en.wikipedia.org/wiki/Electromagnetic_radiation

    That is not possible to answer at this time.

    I'm not sure what you are asking. The wave-like and particle-like properties both determine what happens during this experiment.
    Do you mean physical constants? http://en.wikipedia.org/wiki/Physical_constants

    How about we focus on one problem at a time in this thread.
  10. Feb 24, 2012 #9
    Firstly thanks for still being with me.
    Am still trying to digest the links you have given.
    Thought I read somewhere that electron orbits depend on associated wavelength. So if energy levels are different then associated wave will be longer/shorter. Have also been looking at the number of electrons per orbit, 1st 4 follow the rule - orbit number squared x 2 for maximum number of electrons for orbit 5 onwards there is not much of a pattern, in fact electrons appear to form a new orbit before previous one is full which strikes me as odd.
    Talking about longer waves.
    wiki states energy and momentum depends on frequency or wavelength not sure about
    E=pc and magnitude of momentum vector.
    With frequency I think of the number of waves per second past a point moving forward, oscillating to me means travelling backwards and forwards between 2 points with no actual forward motion.
    Dual slit - if a detector is used particle like, no detector wave like.
    Physical constants will do have seen them called natural constants.
    A quick question although unrelated - is it possible to describe a spiral using pi=3
  11. Feb 24, 2012 #10


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    Yes, each orbital has an associated energy level with it and hence different momentum. Less momentum = longer wavelength.
    Once you fill the 3p orbital, the next lowest energy level is actually 4s, not 3d.
    It's all about energy levels. Look at the pattern of the table here: http://en.wikipedia.org/wiki/Atomic_orbital#Orbital_energy

    The electric and magnetic fields of an EM wave oscillate from + to - and back. The number of times per second this happens is the waves frequency. Take a water wave for example. The water molecules aren't moving forward and back themselves, but are moving up and down, aka oscillating. The wave propagates forward even though nothing is actually moving in that direction. An EM wave is similar.

    All detectors are particle detectors. The interference patterns build up over time as a result of the detection of large numbers of individual particles.
  12. Feb 27, 2012 #11
    I realize that the waves go from+ to - the problem being the forward motion, water waves have a medium to propagate through ie the enegy propagates through the water and the waves being the visible result.
    The question is then if there is no medium (aether) for the waves to propagate through how do they get from the sun to earth without forward motion. I can see it if space is in fact an EM field with infinite stationary wavelength and near 0 energy for the wave to propagate through, this of course would be completely undetectable.
    Have been thinking about momentum and trying to reconcile classical matter with energy.
    My line of thinking - momentum = total energy carried, for matter this is speed x mass and both are variable. For energy it is hf where h is constant and f is variable but we know speed is constant - c so in order to make f constant it has to = 1 where f=c/wavelength so f=c/1.
    Also momentum implies forward motion which waves have not got if they are just going up and down.
    I can picture both waves, rigid ones moving forward and oscillating ones but the oscillating ones need a medium to propagate through whereas the rigid ones dont.
    How do you measure the momentum of a moving object.
    How do you measure the momentum of a wave. I do mean measure not calculate.
    Also matter is rigid to energy is transfered basically all at once although there must be a time variable involved also possible depending on density to allow for the energy carried at the rear of the mess to transfer forward, at normal speeds this would be regarded as 0 though.
    With energy the energy or momentum must be carried uniformly along the length of the wave as the wave cannot be regarded as rigid the total energy can only be regarded as transfered once the whole of the wave has been absorbed and the time that takes will depend on wavelength.
    Another point which relates to this thread is the the speed of light is just as solid a barrier as a brick wall so things could pile up against it at least in theory.
  13. Feb 27, 2012 #12
    Any info on the very long waves?
    Any thoughts on Pi and spiral?
  14. Feb 27, 2012 #13


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    John15, I have been answering many of your questions, but now it is time for you to take the initiative and learn the basics instead of trying to piece together things based on incomplete knowledge. You will never understand these things without a solid foundation. If you read the basics of Classical Physics, Quantum Mechanics, and General Relativity from wikipedia you WILL have a much better understanding than you do now. Almost all of what you have been asking is readily available there if you just take the time to read the articles. We don't mind helping you here on PF, but what you are asking for is to be taught much of all three theories by us, which is not feasible.
  15. Feb 27, 2012 #14


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    Did you ever consider reading what is actually said about waves and photons? It is a difficult topic, intellectually, because it involves thinking along totally non-intuitive lines. You seem to be trying to build a picture of what is happening from a strange mix of inputs, all on your own. You need to read a whole article (e.g. wiki) about the subject without skimming and skipping when you 'think you've got it'. You should also try to answer questions in comprehension exercises that many of the educational web pages carry. Doing it your way could involve a lot of wasted time and you could still leave without the right ideas.

    A spring can't represent a wave. A spring can carry a wave, if you like, and you can have springs, side by side, each of which carries a wave. But that's Mechanical and nothing to do with photons at all. You will not get anywhere if you want it to be just like a mechanical analogy - the GREAT BRAINS at the beginning of the last century had to struggle and argue for decades in order to get anything like a solution to these questions. With respect, you have no chance of doing it on your own.

    A wave is a wave. It behaves according to certain rules and many phenomena can be 'explained' entirely using a wave model. In most (or even all) cases, the only time you really need to involve photons is when the energy, carried by a wave, interacts with a system of charges (electrons and nucleus of an atom or the atoms in a molecule). Chemical terms like 'Orbitals' are not really appropriate; energy states are what count. Any atom / molecule will have its energy defined in terms of a number of so called Quantum Numbers. For an isolated atom, there are not many numbers involved but, for a solid or even a high pressure gas, there are too many numbers to deal with individually. What I'm saying here is that the 'Hydrogen Atom' model will usually let you down if you try to use it as a general way of thinking.
    An em wave of a given frequency will only interact in energy dollops equal to hf and these are the quanta associated with the photons. You just cannot validly look upon a wave as just a shower of photons. A series of peaks and troughs is not a series of waves it's a single wave.
  16. Feb 28, 2012 #15
    I really do appreciate your patience but the answers/explainations I am looking for are not there just assumptions that interpretations are correct.
    I notice you have not commented on my logic about momentum in post 11.
    The fact is you and wiki etc see 1/2 wavelength = 2 x energy. I see same energy x 1/2 time, no amount of looking at wiki will resolve this only an explaination from foundation up. We will leave photons for the time being.
    Take a sheet of paper, the paper represents energy the width wavelength 1 light second to make things easy, fold in 1/2 you get same energy and 1/2 wave when moving at steady speed this gives inverse sqare past a set point yet we have not increased energy but decreased the time it takes to cover a set distance. If you double energy you need to add another sheet of paper which will not give inverse square but 2 x inverse.
    Another problem with many sites is the formula and terms used take derivative for a start a very basic term in relativity but you try finding a simple explaination of it. There is not much I cant understand if it is explained in an undestandable way.
    There are many questions I have and the answers are either not available or deeply buried.
  17. Feb 28, 2012 #16


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    Momentum is not energy. They both involve velocity and mass but the definitions are different.
    You write :" I see same energy x 1/2 time" this doesn't mean anything to me, I'm afraid.

    You still seem to want to impress your initial idea on the reality of it rather than the other way round. I suspect that you don't want to get into the Maths. That really isn't optional for this topic - just go along with this::

    Start with the most familiar wave - ripples from a stone dropped in water. The waves that spread out are carrying energy. As they are on a surface, the energy is spread out around the circumference so that its 'intensity drops off as 1/radius (the circumference is proportional to the radius and the energy is spread evenly around the radius. (I am not concerned with photons or quantisation at the moment).
    Now take a point source of light (equally bright in all directions: 'omnidirectional') and not considering photons - they are not relevant yet. Just imagine it consists of just one frequency and is continuous, to make it easier. The wavelength will be the same all the way out but the energy will be spread out over a spherical surface - hence the energy flux through a window of any size will be proportional to 1/radius squared (because that's the fraction of energy that gets through that particular area). The wavelength and frequency are still the same but the amplitude (the variations in Electric and Magnetic Fields just get less). That's the inverse square law taken care of and it works for anything that spreads out spherically (even the debris from an explosion in space).
    IT SO HAPPENS that they found that EM waves come in quanta and that is the exact amount of energy that EM of a particular frequency can add or subtract from a wave.
    The energy of a quantum E is hf, where h is the Planck constant and f is the frequency. They use the frequency because that is the same wherever the wave gappens to be travelling.

    Important formula coming up:
    where c is the speed, λ is the wavelength and f is the frequency. This is the equation that you need to have in your head at all times and it lets you change between the two, as long as you know the speed. (In space, of course, this is always the same)

    This tells you that you could, if you wanted, say that the energy of a photon can be written
    E = hc/λ

    ALSO, when you work out the Momentum (P) of a photon (literally how much it will knock against an object it hits) that works out out as

    So E of each photon is not the same as P.
    Normally (classically) we talk of momentum in terms of mass times velocity but photons have NO MASS so we define it that different way.

    Joining these two sets of ideas up: a certain amount of total energy will flow through a window in one second. If this is Etotal then there will be
    Etotal / E number of photons passing through the window per second.

    For low frequency EM waves, the photon energy is lower so there will be more of them for any given amount of energy. For Radio Waves, the photons are, say, a millionth of the energy of light photons and, for gamma rays, the photons will each be thousands of times the energy of light photons.
  18. Feb 28, 2012 #17
    I can guess whats going to be said but if energy (mass) stays the same and waveLENGTH and time reduce there are parallels with relativity.
    As I said no amount of reading will give correct interpretation, interpretation only comes with discussion you cannot have a constructive argument with a book or monitor and without constructive argument there is no way forward, I admit the more info available the more constructive the argument and my knowledge may be a bit basic but you build a house out of bricks, the clever part is knowing how those bricks fit together.
    I have seen it said that science seems to have hit a wall and perhaps a new way of thinking is needed, as you have seen I put things together differently it does not matter if I am right or wrong but maybe an idea may connect with someone with the knowledge who says thats not right but it gives me a new idea to work on.
  19. Feb 28, 2012 #18


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    I'm sorry but this is nonsense. It absolutely matters if you are right or wrong. You are putting things together incorrectly because you want to make nature fit your own way of thinking. This is EXACTLY the kind of thing the scientific method is intended to prevent. Your ideas will never work because they are wrong fundamentally and no amount of rearranging of equations or anything else will change that. Anyone who claims science has hit a wall and "needs a new way of thinking" is a moron who doesn't understand anything about it.
  20. Feb 28, 2012 #19


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    @ John
    Just what do you mean by that first sentence? Classical wave theory has nothing at all to do with relativity. It pre-dates it by a hundred years or more.
    You write as if you have brought something New into the subject. I can see nothing more than a few misconceptions on your part. You cannot seriously believe that you are the first to try and see things from your particular perspective. Science has not recently hit any wall in this area. That wall was cleared up about a hundred years ago.
    Your problem is that you have not really accepted what a good fit these 'bricks' make for this particular structure. You are needlessly bringing mass and energy together at this stage and, unless you can go along with what others have already sorted out then you are 'on your own'. There can be no 'constructive argument' if you are not starting with some common ground.
    It is not uncommon for people to think they have stumbled on something really 'significant' but I can't think of a single example of a truly significant bit of Science coming from someone who made it up all by themselves. It was only when they could appreciate fully what was already established that they made significant 'next steps'.
    I don't think you are aware enough of the true complexity of all this. If you don't get the simple maths then you are onto a loser, I think.

    [Edit: Sock it to him Drakkith!]
  21. Feb 29, 2012 #20
    The bit about science hitting a wall or need new ideas is not my own, it came from a book by lee smolin and a science prog on tv recently, said by someone in the scientific world.
    As far as right and wrong go you cannot know or find out what is right without knowing what is wrong its a process of elimination.
    My view on wavelength has still not been answered. If energy doubles as wavelength halves then eventually you will reach a point where the wavelength is so short it will have the energy equivalent if not of the universe then a supermassive black hole, energy cannot be created or destroyed so how can it be doubled just by shrinking a wave.
    While I accept time and length dilation I have always wondered why they should exist, there is absolutely no logical reason why time should change because you are simply going faster.
    You have seen my line of thinking all I ask is that you show me where I have gone wrong.
    Please take E=hf apart and explain it step by step, we are talking about single wave not a line of them which is what frequency infers ie waves (plural) per second.
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