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The nature of light.

  1. Jan 11, 2005 #1
    This topic may have been covered but I am not sure how to search for it, what phrase would I use? So my apologies if I am going over old ground.

    I am have great difficulty with light and its nature.

    Recalling some very basic physics, light behaves as both a particle and a wave.
    true or recanted ?

    If light is a wave what makes it take on a particle status?

    Does the wave 'collapse' to a particle?

    Consider a far distant star. The light from it has travelled many trillions of kilometres. Why is it that the person next to me can see the same star as me.

    Surely as the light expands from the star large gaps must appear so that if a light particle hits my eye, my friends eye is missed.

    Draw a circle and then draw lines from it radiating out, the further from the circle the greater the distance between the lines.

    Now try to draw enough lines so that there is no space between the lines

    Now extend these lines a few trillion K's. without creating spaces.

    Can any one please explain this?
  2. jcsd
  3. Jan 11, 2005 #2


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    true. The dual nature of the photon is still supported by all experimental data so far.

    Perhaps the best example is your microwave oven (assuming you have one). The reason it works for heating food is because it sends out EM radiation (photons) at a certain wavelength, and certain particles like water and some proteins oscillates in resonance to these frequencies. In other words, they "ride the waves" up-and-down and this heats the food. But what about those holes in the top of the microwave to vent air? When a wave traveling through water hits a barrier, and that barrier has holes in it, small portions from the original wave pass through the holes and continue beyond the barrier. But photons are indevisible, so if a hole isn't big enough for the entire photon to pass through, then the entire photon stops or is reflected, like a particle.
    Again the analogy of a wave traveling across water is useful here. If you throw pebble into a pond, waves begin expanding out from the point of impact in all directions. As the waves spread themselves out over broader and broader area, the loss of energy concentration shows itself as a decrease in amplitude (just like a star gets dimmer as you get farther away), but the wave never does start to form "gaps".
  4. Jan 11, 2005 #3
    Yes I considered this.

    So the wave front expands as it leaves the star.

    But light does not expand. unlike low frequency electromagnetic waves, light is unidirectional. ie: a lasar

    So the rippling pond sounds great but really is entirely inaccurate, is it not?
  5. Jan 11, 2005 #4


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    I will simply give you a summary of the nature of light, and if you still have more questions ask.

    Light travels at c which is 299792 km/s. No slower, no faster, always at c. Light is made up of massless particles called photons. It is right to say that light is both a wave and a particle, this is called wave-particle duality. Everything on the electromagnetic spectrum is considered light. They are show in form of waves. A wave has frequency and wavelenght. So for example, the color violet will has a higher frequency and smaller wavelenght then the color red. This is all waves. But if you look deeper into light you will see that it also acts like particles. For example, when we shine light on metal, electrons are emited. Why? Because (in a nutshell) single particles called photons are hiting them out of place. Another example, is when we heat an atom, the atom gains kinetic energy and emits a photon to regain it's normal state, called eigenstate. We feel the photon (energy) as heat. If we stop heating, the atom cools and is now confortable again and everything feels normal, no more thermal energy. (2 gamma photons are considered energy) So basically when you deep into a wave of light you see photon particle packets.

    Like you said, a star millions of light years away is emmiting photons in every direction. On earth, we pick up those photons with our eyes (someone next to you can do the same) and we see the star. But because light has a finite speed, we are looking at very *old bunch of photons emmited by the star. In other words, we are seeing the star millions of years earlier. The futher the star is, the younger we are seeing it.

    But something happens to light as it travels. If it is going through a gravitational field (which are everywhere in space) It will lose some of it's energy. But it will always travel at c. This is called red-shifting. Let me explain. When light waves always want to travel at c. But when something is trying to slow them down, like a gravitational field, they will use up some of their energy to continue traveling at the exact same speed, c. When a wave loses energy, it's wavelenght gets bigger, making it more red. The same happens as blue-shifting, not speeding up light, but giving it more kinetic energy, more frequency, making it more blue.

    *Light or photons dont experience time. They never get old, are never young. This is why time slows down (from your perspective) as you speed up, you are getting closer and closer to not experiencing time. Though you can never achieve this because of Einstein's famous E=mc^2.

    -look up the electromagnetic spectrum and you will have alot of understanding of light.
  6. Jan 11, 2005 #5


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    oops look like im too late. lol
  7. Jan 11, 2005 #6
    Or as I have just pondered do all the particles that leave the star at the same time and on the same plane join together to create a massive light wave which is the sum of its parts.

    If so what happens when a heavy body intervenes?

    observation tells us the wave simply continues, except for the section that was absorbed or reflected by the heavy body.

    again I am left to wonder what happens when light reverts to a particle state. is its wave front collapsed? or is there an infinite number of potential particles in any given wave front?
    Last edited by a moderator: Jan 11, 2005
  8. Jan 11, 2005 #7
    My last message overlapped

    DB Your kind response missed the point. again I refer to the lines drawn from the star.

    for my friend and I to see the same star each must recieve a number of photons these particles of light leave that star at the same time (when is irrelevant)

    That means the particular point of the star that emmitted those photons must have emmitted near infinitely massive amount for us both to see the star. as stated the further away we are the weaker the strength of the light, or is it more accurate, the fewer the photons hitting our retina, which tends to support the diverging lines of light concept.

    Therefore the amount of photons leaving a star MUST be immensely massive so massive its beyond comprehension (mine anyway).

    I hope this brings my quandary into more perspective.
    Last edited by a moderator: Jan 11, 2005
  9. Jan 11, 2005 #8

    You are confusing several things here. Wave/particle duality does not mean that you can pick and choose how you want to see it. The nature of light in a medium is a wave. When it is detected it hits a single spot (like your retina). But in any star there are billions of waves of light being emitted. Whether or not these waves have a fundamental particle nature is not known for sure. And anyway, the star is composed of billions of particles emitting forms of EM radiation. To say that "the particular point of the star that emmitted those photons must have emmitted near infinitely massive amount for us both to see the star" is missing the point on several levels.

    Does that make sense in any way ? If it does make complete sense then you're missing the main issues, but does it at least answer your question ?

  10. Jan 11, 2005 #9
    I am obviously missing the point otherwise I would not need to ask the question

    But your response does not address any of the points I must be missing. (if I knew what they where I wouldnt need to ask)

    The duality of light is pertinant. how does light react with the retina? at the point of contact does it take on its particle (photonic) role thus allowing it to impart its energy into the sensory organ, so that I may see it.

    If this is the case then myself and my neighbor who must have intercepted the same wave do we share a photonic particle or is the wave infinitely divisible into particles, so we both get our own seperate photon.

    perhaps better put as... what section of the wave collapses into a particle?

    Does the wave choose whose eye it goes into. thus I get some particles and my neighbor gets other particles. but this happens so often we seem to be seeing them at the same time.

    DB I will ask questions obout the nature of light and time at another time as I have thoughts on that as well.
  11. Jan 11, 2005 #10

    The evidence suggests that only one of you will ever "see" any particular "wave". But remember there are millions of them. Its an interesting question though, as if both you and your friend were at a 45 degree angle from the star, and placed so that the "wave" reach both of you at exactly the same time, which of you would get the "photon experience" on your retina ? I'm sure the more knoledgable here will be able to answer that one :)

    This is a flawed understanding of the copenhagen interpretation. The energy that is the wave disipates in the retina in a single point. What "section of the wave" are you talking about ?

    If it hits your eye first then you see that particular wave. Remember a star is not a single particle that emits a single wave :)


    PS. Don't take that as gospel - I'm hoping people here will cut me down. Argument seems to me to be the best way to learn. Of course you have to be receptive to the idea that your own personal understanding is probably flawed in many ways!
  12. Jan 11, 2005 #11
    In response to benpadiah...

    So a photon actually goes nowhere at all. it never leaves the surface of the sun.

    Its energy is simply rippled over the ether until it reaches me. that energy is then converted into a new photon that interacts with my eye and I see it.

    the same as a water particle in a wave never moves forward it just oscillates up and down as the wave passes through it.

    Also the same way as if I placed a stick in your eye and whacked my end of the stick with my hand. my hand never actually goes near your eye but the energy imparted ripples down the stick (at near light speed) and converts into a black eye at the other end.

    please excuse my graphic analogy

    This means that the vertical oscillation of the energy front is absorbed but the horizontal plane of the wave front is not effected (except where it is intercepted).

    I use the terms vertical and horizontal loosely here I hope you understand my meaning.

    So the next question for me is what makes up the stick between the star and me.
    This is the ether they speak of.
    What is it? it obviosly is not a collection of atoms like a real stick yet energy travels along it without loss (apart from the thinning effect).

    Sorry SimonA you got in while I was writing, but I think you will see I am learning, hopefully at lightspeed :)
    Last edited by a moderator: Jan 11, 2005
  13. Jan 11, 2005 #12

    Kane O'Donnell

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    No no no no no!

    I don't know what benpadiah is really talking about, but it sounds very, very wrong to me.

    Light comes in lumps of electromagnetic energy called photons. A gazillion photons together behave very much like a single wave.

    The fact that light is inherently lumpy has been shown time and time again to the point where it is effectively beyond doubt that photons exist. In fact, we have tools like single-photon microscopes that *depend* on light being lumpy.

    You and your friend see the star because, as you surmised, many gazillions of photons are emitted by a star every second, and you're seeing different photons. A relatively weak Helium Neon laser emits around 3.2x10^15 photons *per second*, and that's a laser (very narrow emission spectrum). A star emits many billions of times more.

    A single photon does exhibit wave behaviour, but it's subtle. You shouldn't think of a photon as a spherical EM wave, where the wave exists *at all points on the wavefront* simultaneously (ie if we had detectors separated in space we could simultaneously measure the wave in different places). A photon can only *ever* be detected in *one* place, but the actual place where it IS detected is only known up to a probability at any given time.


    Kane O'Donnell
  14. Jan 11, 2005 #13
    Hi Kane

    Do you consider that the fact light is observed to be "inherently lumpy" (quantised) is a property of the qantised nature of the emitter and the detector in terms of them consisting of quantised matter ? If you don't, then do you have any suggestion for some kind of evidence to describe why you believe that any form of EM has a "packet like nature" in itself, seperate and independant from the quantised nature of its emitter and detector ?

    Last edited: Jan 12, 2005
  15. Jan 12, 2005 #14
    I like it when people challenge me. Its an opportunity to learn more. If your only response is "explain", and "how rude", then you don't really seem interested in any of this and I wonder why you are here.

    I myself tend to annoy the moderators here and they may well agree with you that I'm being "rude" here. But I think I was being honest, and I think you are talking nonsense just for the sake of it. If not, you would be happy to describe youngs slit experiment, and the adition of the detector in front of the slits, and what the results of that mean to you. If you can't do that at all, then don't you rekon you should be asking questions rather than answering questions of people who seem to me (in my admittedly faurly ignorant understanding of the subject) to have a better understanding of it than you do ?
  16. Jan 12, 2005 #15
    Ok Enough with the flames

    I was able to understand what Ben was getting at. what he wrote was clearer than others.

    I need to review and understand the orther posts b4 I respond further
  17. Jan 12, 2005 #16


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    Unfortunatly what ben wrote was so confused as to be wrong. Sorry I did not catch it sooner.
  18. Jan 12, 2005 #17


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    I honestly think Kane and Simon are absolutely right, simply there are millions of waves for everyone to see. And the further they originate from, the younger the object (star) appears to you.

    -I don't want to nag, but isn't it the iris of your eye that interprets color (photons)?
  19. Jan 12, 2005 #18
    DB The iris is the opening at the front of the eye.

    The retina contains the cells that detect light


    I will try and do some basic math to illustrate my point

    Question: Can 2 photons occupy the same space and time ?
  20. Jan 12, 2005 #19


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    Ahh, thanks the iris controls how many photons enter, the retina responds to them. Thank you.
  21. Jan 12, 2005 #20


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    Yes, multiple photons can occupy the same space, there is not limit. This is the reason we see white light, it is the superposition of many photons.
  22. Jan 12, 2005 #21
    Sorry to ask more questions about the eye, but, are the rods and cones in the retina adapted for recieving waves or particles ? .. Surely evolution, if anything, should be able to determine the nature of light ?
  23. Jan 12, 2005 #22
    I wouldnt take this to a science fair.

    The colour of light is a function of its wavelength. it does not mean that the photons overlap.

    Does the eye see photons or waves ?
    How would one determine this. because light is both a wave and a particle and we don't know at any given time which it is I doubt we could discover this.

    Does an aerial see electromagnetic energy as waves or particles ?
    what excites the electrons in an aerial ?
  24. Jan 12, 2005 #23


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    I agree with Integral here. Light is also a function of energy. White stars (O) for example, are the biggest, most luminous and powerful. So it would seem to me that with such a powerful emision of energy (photons) as light, it could lead to photon waves overlaping creating white light.
  25. Jan 12, 2005 #24
    My question was probably not clear I meant photonic particles not waves
    can two photon particles occupy the same space?

    I know two waves can occupy the same space.
  26. Jan 12, 2005 #25
    That's what this place is for. The fact that you tried on your own first is a good thing. :)

    First of all, you should understand that everyone- including Nobel Prize-winning physicists- has great difficulty with the nature of light. Let me explain why.

    You are used to a world of objects that you can see and touch, and that have many characteristics. The smallest thing that you can see or feel is, on the level of a photon, incredibly huge and complicated. Billions and trillions of photons could fit in the smallest speck of dust you can see or feel. As a result of this, you are used to things that have many characteristics. At minimum, they have billions of atoms in them. Because of this, their behavior is not the behavior of a single thing, like a person; it is a collective, group behavior like a huge crowd of people. You have never encountered an object that was like a single person in that crowd, and you never will.

    So when someone describes the characteristics of a photon to you, what they are describing is something that you quite literally cannot imagine; you have never encountered anything like this before, and you never will. You can only make up models of it in your mind; it's like a wave, or it's like a little ball, or its like a kind of little patch of space that has something weird about it. But it isn't really any of those things- it's a photon, and we never have seen one and we never will. And this is not merely true of photons- the same is true of atoms, and electrons, and neutrons and protons; in fact, all of the little things that are flying around making everything around you up are like that.

    So first of all, you must get rid of your preconceptions that are built on a lifetime of experience dealing with complex objects in the world around you, because if you continue to think in those terms, you will never understand this at all. Actually, you never will and never can completely understand or comprehend it; it's starkly impossible, because you have never encountered anything simple enough to be like a photon, and you never will. But at least you can make up a model of it that will help you understand how it behaves. And remember that no one can understand it- all they can do is try to imagine it, just as you will do, and try to understand its characteristics by making models in terms of familiar things, just as you will.

    So, now, empty your mind of preconceptions and we will see what we can do to give you an understanding of what things are like in the realm of quantum mechanics.

    Well, no, not exactly. Light behaves as if it were made up of waves under certain circumstances, and as if it were made up of particles under certain others, and in some very confusing ways that are something like an amalgam of both under some very special circumstances.

    Light doesn't take on "particle status" or "wave status." It is what it is, and what it is is indescribable. Sometimes, we detect light behaving in ways that we can interpret as being wave-like, and other times particle-like, and in a few special circumstances like nothing at all that we have another name for than "photon-like." But to think that it's "being a wave" or "being a photon" is wrong. It's just being light, all the time; we just are better able to understand what it does under different circumstances by using those analogies.

    Now that you have no preconceptions, let's try to give a more substantive answer. Our best models of light when we deal with it in terms of very small amounts of it are particle models, and we call those particles "photons." We have proof that there is some sort of "granularity" to light; it isn't just a continuous wavelike thing all the time, we can see that there is some sort of minimum amount of light that we can emit or absorb, at least for any one frequency. The "size" of one of these "granules" of light, which we call "photons," is very, very, very small. It is also dependent on the frequency of the light. Remember that radio waves, and microwaves, and infrared, and visible light, and ultraviolet, and X-rays, and gamma rays, are all this same thing; the only difference between them is their frequency. I have given you the names of the big parts of the electromagnetic spectrum in order from lowest frequency (radio) to highest (gamma). So the point is, that the radio photons are the smallest, and the gamma photons the biggest, at least in terms of the energy they carry.

    If you want to know the proof of the existence of these photons, then you should read about Max Planck and the quantum theory of black body radiation, and about Einstein's quantum theory explanation of the photoelectric effect. There is a very good explanation in Isaac Asimov's The History of Physics, Walker, 1984. You can also get a little information from the links above, which are to Wikipedia; in the black body article, its about the third and fourth paragraphs, and in the photoelectric effect article, look for Einstein's name, where the explanation of the effect by the quantum theory is described. It's worth noting that Einstein won his Nobel Prize in physics for his description of the photoelectric effect, not for the Theory of Relativity.

    Getting back to our subject, when we deal with large quantities of photons, it's better to think of them as waves. And it turns out that the waves have real, physical existence as well- that's how we can speak of frequency above, because this is a characteristic of waves. It has wavelength to go with the frequency, as all waves do, and a speed of propagation, and in fact the relation among these is the same as it is for all waves. The most important reason that we know light is wave-like is because it exhibits a behavior that we know can only be shown by waves: interference.

    So here we have something that interferes like a wave, yet is emitted and absorbed like a particle. So: is it a wave, or is it a particle? Neither. It is light!

    This is actually a rather astute question; you must have been reading about the "collapse of the wave function." I'm going to introduce you to another way to think about things, which will help you some with this if you can get your mind around it.

    A light wave is an oscillation of the vacuum. We don't really know what vacuum is, but we know that it can oscillate, and that these oscillations represent the probability of finding something- an electron, a photon, or whatever, at a particular location. The speed at which these oscillations travel through the vacuum is the speed of propagation of that thing- but until something happens, an event of some kind, the thing isn't really there; it's got a probability to be there, but it also has a probability to be over here, or over thataway. Once an event occurs, then we know right where the thing is- but that's all we know, and we only know it for that moment when the event is happening. If the oscillations represent photons, then they travel at the speed of light. When they encounter an obstruction, that is an event- and because an event has happened, the thing that those oscillations represent, a photon in this case, suddenly has a precise position. This change from an oscillation to a thing is called the "collapse of the wave function," and the oscillations are "probability waves."

    I know this seems kind of confusing, but that's as well as anyone understands it. Including nuclear physicists. What they have that you don't is a bunch of math that they can use to predict the exact probability that the photon will show up over here or over there when the event happens- but remember, they don't know until the event happens whether it actually will be over here, or over there, or maybe even over yonder; all they know is the probability of it winding up in those places. And one of the key things about quantum mechanics is that it says that that is not merely all that we can know- but that those probabilities are all there is to know. The photon doesn't have a position until the event occurs- all it has is a probability.

    You vastly underestimate the number of photons involved. "Trillions" is a number so large as to be unimaginable- but it is also a number so small compared to the photons the star is putting out every second that it isn't even big enough to be microscopic. It's like comparing a kitchen match to the Sun- or maybe even to a million suns. So out of that truly astronomical number of photons, maybe ten or fifteen per second are hitting your retina, and about the same for your friend (unless s/he is wearing dark glasses). There are plenty to go around. You have to get a lot farther away or be a lot dimmer than any star you can see in the sky is before there aren't enough photons hitting your eye for you to see them.

    It might help you to know that there are about 6,000 stars in the sky that you can theoretically see- on a very dark, very clear night, far from the city lights or any lights at all, after you've let your eyes adapt for an hour or so in pitch blackness. And that's in the whole sky; at any one time, you can only see perhaps 2,000 of them. Well, within the sphere that those 6,000 stars make, out to the farthest one- there are over five million stars! So you can only see about one star out of a thousand. Every star you can see is either unusually close or unusually bright.

    No. The star makes so incredibly many photons that even at hundreds of light years away, there are plenty for both of you, and they are plenty close together.

    There are so many photons that there aren't spaces more than a few thousandths of an inch where no photons will fall in a second. Your analogy is bad, because you are thinking of photons as relatively large things; they are actually very, very, very, very, very small, smaller by far compared to you than you are compared to the whole Earth, or even the whole Solar System.

    If you want to learn more about this pretty quickly, you can get a small, short, inexpensive book by one of the most brilliant physicists who has ever lived- Richard Feynman. But you don't have to be a genius to read it; Feynman wrote it for everyone, and it is very simple. That is part of his genius. The name of the book is QED: The Strange Theory of Light and Matter, and it was published by the Princeton University Press in 1985.
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