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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.
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