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B Composition of Light

  1. Feb 14, 2018 #1
    Hi everyone,
    I have a few questions about the composition of light:

    First, what is it? Is white light the result of all color wavelengths present in an area?
    Second, if so, then why is there no interference in waves of light (or is there)?
    Third, if photons all travel at the same speed, then doesn't this imply that the amplitude of a red wave would be greater than the amplitude of a yellow wave?
    Fourth, why is a photon absorbed or reflected? In other words, what happens, in terms of energy, that causes a photon to bounce, be absorbed, or to have partial reflection/absorption?

    Thanks.
     
  2. jcsd
  3. Feb 14, 2018 #2

    sophiecentaur

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    Hi
    Well, you seem to have managed to ask almost all of the questions about light possible. :smile:
    I think that you need to read more around this topic and get some basic in before you try to construct actual questions.
    Here is one of the earliest hits I found from a google search into "the nature of light". Try reading it and follow the links too. Don't skip things that are a bit boring or tedious as they may be vital for your understanding. There are many other links that Google will give you. Hopefully, you will be able to spot the ones which are mainly BS and there are several of those on the web (as with all subjects).
    I am assuming that you don't want to buy an A Level text book but that's really what you need.
     
  4. Feb 14, 2018 #3
    Hi sophiecentaur,
    I've read that page as well and have done some basic googling to little avail. If you can recommend an A level text, I'd consider the purchase (and honestly, prefer it so the researchers can get paid!). But, I have not been able to find anything that directly addresses these particular aspects of the functioning of light. Logically, at least the part about amplitude makes sense to explain why redshift occurs in space, being basically that red light is left behind as the beams of light travel with indigo (or more likely sky blue) leading the beam (or something beyond our visible spectrum). But yes, I'm interested in hearing more about this and thank you for any recommendations.
     
  5. Feb 14, 2018 #4

    Khashishi

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    Jackson, Classical Electrodynamics is a standard text on electromagnetism (and therefore, light).
     
  6. Feb 14, 2018 #5

    sophiecentaur

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    There are many different A level text books and any one of them is worth using as long as it is recommended by one of the examining boards. I wouldn't recommend a 'course book' that's aimed at a modular course because they can be quite limited. You need a full text book which is aimed at a standard Physics A Level course - it can be pretty much as old as you like because things like Dark Matter do not come into basic Physics. Plenty of time for that later.
    You clearly have some large gaps in your knowledge of light. It is one of the most important facts that the speed of EM waves in space is the same for all frequencies and for all observers under all conditions. I can't think where you got that stuff about slow red photons. I have frequently made the point on PF that learning of Physics (or any other Science - or in fact any subject) is not achievable by simple Question and Answer on Science Forums. You have to start at the beginning and work through and you can then avoid the sort of mis information that you seem to have acquired somewhere. What have you actually read so far?
     
  7. Feb 14, 2018 #6
    My official training in physics is basic first year collegiate physics (Big10). I come from a philosophy background and started this research in relation to aesthetics and color. The part about the standard speed of EM waves is kind of where the question lies. Basically, is what we call "the speed of light" the speed of a photon or the speed of a wave? And, if it's a wave, then which one? A red one? A green one? This is the crux of what I am really trying to understand. As a result, if the speed of photons is what we mean when we say that the speed of light is constant, then it should follow that, even though the speed of the photons are constant, in order for the frequency to change from color to color, the amplitude must also change. This means that a photon would have to travel along a longer path over a shorter distance to make red at the same speed. This is what is confusing to me about what we mean when we say "the speed of EM waves" or "the speed of light." Sorry, but I don't remember the exact text titles and authors of the books.

    Khashishi, thank you for the recommendation.
     
  8. Feb 14, 2018 #7
    To clarify, I'm imagining the photon traveling in a wave pattern (like a sine wave). Maybe that is the issue?
     
  9. Feb 14, 2018 #8

    Khashishi

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    By the way, Jackson's book won't really tell you what a photon is, since it is a classical treatment. Nevertheless, you really should understand classical electrodynamics before you jump into quantum electrodynamics. It's simple enough to picture light as a wave, but don't bother trying to picture a single photon. It's just not something that can be pictured.
     
  10. Feb 14, 2018 #9

    phinds

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    You seriously misunderstand light. ALL light travels at c. Different wavelengths register on the human retina as different colors. Light travels as a wave. Photons are the result of an interaction of that wave with an object.

    Now read post #5 again and follow his suggestion. This Q&A stuff isn't going to cut it.
     
  11. Feb 14, 2018 #10

    Drakkith

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    Yes, that's a widespread misconception. Photons do not travel in up and down or side to side wave-like motions. What makes an EM wave a wave is the alternating electric and magnetic field vectors. These give the direction and amplitude of the electric and magnetic fields at any point and it is these which alternate back and forth, changing the direction and strength of the force that would be applied to a charged particle.

    Photons cannot even be said to travel in a classical motion. They are not tiny point-like balls that move in straight lines or sine waves. They are the quantized interaction of the EM wave with matter.
     
  12. Feb 14, 2018 #11
    Thanks for the information. I'll have to see what the University bookstore has, or order something, or go back to school. We'll see.

    Just for kicks, the question that I was thinking about that began this was this:

    What was the order of colors that came about at the creation of the universe? Darkness into...

    Any takers?
     
  13. Feb 14, 2018 #12
    Isn't this still being debated though? Isn't that what string theory is about; the idea that matter is made of vibrations (ie. sound and light)? If true, doesn't that also imply that the interaction with matter isn't so much of an interaction but the changing of state of energy from a vibration to matter? That would imply that the EM wave is a different state of energy and that photons are the result of that change in state from vibration to matter.

    Going to the bookstore...
     
  14. Feb 14, 2018 #13

    sophiecentaur

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    Colours only existed after Human Beings developed their tristimulus colour vision - a long time after the beginnings of the Universe.
     
  15. Feb 14, 2018 #14

    sophiecentaur

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    Don't even go there until you have a proper grasp of Physics that is actually based on experiment. You are managing to show a serious contempt for the several millennia of Scientific Learning by treating it all as word salad.
     
  16. Feb 14, 2018 #15
    This is kind of like the tree falling in the forest question. Do colors exist if nobody is there to see them?

    Also,
    Please accept my apology, but I think Newton would be asking similar questions.
    It's not contempt at all. It's an effort to understand things that are nearly(?) incomprehensible and we use words to exchange ideas.

    Now, I'm going to the bookstore, for real. Again and sincerely: apologies and thanks for the information.
     
  17. Feb 14, 2018 #16

    sophiecentaur

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    Newton did all his homework before shooting off his mouth. Remember his statement about standing on the shoulders of giants? He would, in 2018, be asking the sort of questions that showed he was up to date with the current state of learning.
    Maybe not intended but you are constantly undervaluing it and implying that you are just round the corner from understanding it all. Judging by your words, it is a lot harder than you could even dream of. "Understanding" comes from hard graft and not from a few PF posts with questions.
    Just get down to the bookstore - or even eBay. And good luck with it.
     
  18. Feb 14, 2018 #17

    Drakkith

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    Colors aren't physical, quantifiable concepts. They are subjective concepts that exist purely in the mind of the beholder. My stepfather is red-green colorblind and he absolutely does not see colors like you and I do. Even "normal" people's perception of color is subjective, with appreciable differences existing between many people. So the question of which color came into existence first is a question which cannot be answered.

    In addition, the extremely early universe was filled with a very hot plasma and any EM radiation present would have been well into the gamma ray range.

    Not as far as I know, no. Any theory that successfully replaces quantum electrodynamics would still need to quantize the interaction between the EM wave and matter. This is because any future theory would have to make the same predictions that QED does, which requires a quantized interaction.

    String theory states that point particles (elementary particles) are made out of 1-D strings whose different vibrational states determines its properties. But note that it is the strings that can have different states, not energy. Energy cannot have a state, as energy isn't a physical object or a field or something.
     
  19. Feb 14, 2018 #18
    So, if we were using an analogy, it'd be like one end of the string is a liquid and the other end a solid?
    It's seems more likely (to me) that they'd be shaped more like a rubberband where one side might be vibrating differently than the other and that the differences might cause the variable states.

    As for the original post, I went to the bookstore.
    Jackson, chapter 10.1-5 similarly describes what I was talking about down to theta=wavelength/linear distance. What is not explained, and what I was asking, is theta related to the amplitude of the wave being addressed. I liked in 10.2 that Rayleigh had similar questions about why the sky was blue.

    To sophiecentaur,
    I've also read Standing On the Shoulders Of Giants. I may not have a degree in physics, but I have read Hawking, Riemann, Newton, Liebniz, and many of the greatest thinkers in the past few thousand years. I am aware of the many contributions made over the centuries, the longstanding traditions, and the amount of time it takes for even the smallest steps of progress. However, I can also remind you that science itself is standing on the shoulders of people like Plato and Aristotle and has still not fully contended with Popper, Kant and many other metaphysicists. I think that the issue is that my language sounds crude because my understanding of what a "vibration" or "frequency" (or other scientific term) might be is likely much more shallow in terms of scientific rigor than a practiced physicist, and in some cases, our definitions may vary. Those differences, however, do not negate that what I say might be partially correct (or not), but that our understanding of the subject matter is very different. I'm not "shooting off my mouth." I was asking people who I expected to know more about the subject because I know that I don't know everything about everything. So, again, apologies and thank you for the information.
     
    Last edited: Feb 14, 2018
  20. Feb 14, 2018 #19

    Drakkith

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    No, not at all.

    Different sides of the string can't vibrate differently. A string from string theory is analogous to a classical string. If you look at a guitar string, for example, each string only supports certain frequencies that it can vibrate at for a prolonged period of time. If you slightly increase or decrease the frequency, that vibration will quickly die out. Plucking a string at one end causes the entire string to vibrate. You cannot have the two ends of the string vibrate differently. Similarly, strings in string theory have certain "frequencies" that they can vibrate at. Others are not supported. We can say that the frequencies at which the string can vibrate are quantized.

    I assume you mean this question:

    As far as know, the amplitude of two EM waves is equal when they have the same energy density. For two waves of different wavelengths, this means that if you measure the wave with a longer wavelength you will observe more photons per second than if you measure the shorter wavelength wave. So no, there is nothing that, in general, means a shorter wavelength wave will have a lower amplitude than a longer wavelength wave.
     
  21. Feb 14, 2018 #20

    bhobba

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    It is.

    But you should build up to it - I would start with Susskind first:
    https://www.amazon.com/Special-Relativity-Classical-Field-Theory/dp/0465093345

    Then the good old Feynman Lectures which are available online fro free:
    http://www.feynmanlectures.caltech.edu/

    Although IMHO it's so important as a resource I would buy a copy - of course I have and read it every now and then.

    Then Jackson.

    Although I prefer one not that well well known, it's a bit different but IMHO in a good way - Schwinger:
    https://www.amazon.com/Classical-Electrodynamics-Advanced-Book-Program/dp/0738200565

    Jackson dismisses as silly things that greatly interest me, such as how to reasonably justify Maxwell's Equations. I think its important to have an 'intuitive' idea why its true - but Jackson is in the camp - its experiment stupid - not pretty arguments.

    But for questions about Photons start with Feynman's good old QED - The Strange Story Of Light And Matter.
    https://www.amazon.com/QED-Strange-Theory-Light-Matter/dp/0691024170

    Thanks
    Bill
     
    Last edited: Feb 14, 2018
  22. Feb 14, 2018 #21

    bhobba

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    Jackson is considered THE textbook, but its at graduate level - you need to build up to it. See my other posts.

    Read the Feynman Lectures - it will likely answer any questions you have - do not go for the sledgehammer of Jackson to start - that's like crucifixion - first offence - not the best idea.

    Thanks
    Bill
     
  23. Feb 15, 2018 #22

    sophiecentaur

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    @bhobba Those suggests are fine but the OP needs to be sure of the basics of A level (and earlier) Science plus the associated maths. There's no point in trying to run before one can walk. People often try to read the more advanced stuff as it it's Popular Science and then misinterpret things.
    PS The OP should learn that λ (the usual symbol for wavelength) is Greek Lambda.
     
  24. Feb 15, 2018 #23

    bhobba

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    Of course Jackson and other advanced books mentioned is fine. Its just this is a beginner level thread. The OP has just done first year standard physics - you could possibly get by and if you can its of course a no brainer suggestion. The trouble is as you mention, has he the basics down pat well enough to do it. Even if he does I personally would still do it in stages - Susskind, then the Feynman Lectures, then Jackson. Just my personal way of doing things. I am at the moment reacquainting myself with GR. My favorite when I was really into it was Wald but its the most advanced. I am doing it in stages - Soper - Classical Field Theory, Ohanian - Gravitation and Space-Time, MTW, then Wald. I could have gone straight to Wald - but my view is its not a race - taking a slow, easy gradual approach makes for an easier ride. And along the way the OP may find he doesn't need the more advanced texts to answer his queries.

    Actually if the OP wants EM in the context of general field theories Soper is a good choice.

    Thanks
    Bill
     
  25. Feb 15, 2018 #24

    sophiecentaur

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    I was trying to make realistic suggestions of stuff the would be appropriate. Have you actually read his comments?
     
  26. Feb 15, 2018 #25

    bhobba

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    Point taken. I certainly didn't make it clear why I was suggesting it. It was in relation to:

    I was thinking with a philosophy type background he would want to understand the foundational principles it is built on - ie field theory in a general sense.

    Thanks for pointing out I should have been clearer - context of suggestions is always important.

    Thanks
    Bill
     
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