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I Why are electromagnetic waves not visible themselves but...

  1. Feb 20, 2016 #1
    Why are electromagentic waves not visible itself but visible if they are reflected by an object ?
    Assuming a source of light emitting photons. This waves are not visible in the dark. If they end up on a surface of any object which "reflects" the light we start to see the object. What happens at the object, that we can see the light or the object which is kind of the same electromagnetic wave emitted by the source? Is the "reflected light" still an electromagnetic wave? How can we locate, where the reflected light comes from? Even if the light travels "straight" or takes the way of the shortest time, it could still be closer or far away. How do we now, how far the object is? Would the same object, whichs distance is shorter to us send out the same light or information?
     
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  3. Feb 20, 2016 #2

    Vanadium 50

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    The light has to hit your eye.
     
  4. Feb 20, 2016 #3

    bhobba

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    None of the things you mention depend on QM. It should be posted in the classical physics section.

    That said they are very basic and I am surprised they need answering.

    Thanks
    Bill
     
  5. Feb 20, 2016 #4
    Ok either I'm really very confused about this or I don't know how to write up my question neither in my language nor in english.
    I do know about waves, wave reflection and also, that the waves have to reach the eyes.
    And I posted this in this forum because I started thinking about this, when I was reading about the franco-hertz experiment. If two vacuum tubes used for the experiment have a different distance to us and emit not a continuous but just a short light impulse, how does this wave bring the information to the eye, that we can locate the source of the wave. It seems that I'm just really confused and it's more a biological question as how our brain can handle the information, that the nearer wave reaches earlier than the other one or so.

    So maybe sorry for spreading my confusion all over the internet.
     
  6. Feb 20, 2016 #5

    Nugatory

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    General Physics is as a good a home for any for this thread.

    There may not be any really good/satisfying answers to the question you're asking, but you will get a pretty good overview of the current understanding if you do two things:
    1) Get hold of and read Daniel Dennett's book "Consciousness Explained". (The book is good even though the title promises more than the book delivers).
    2) Google for criticisms of this book, and read these too.
     
  7. Feb 20, 2016 #6

    Drakkith

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    Well, I think the first thing you need to do is to separate your question into two parts. The first and easiest to answer is how your eye detects light waves in the first place. This in turn can be broken down into how the eye focuses the light, and how this light is detected.

    I'll start with the latter, the detection of the light. The basic idea is that there are cells in your retina that are sensitive to visible light. These cells have special structures which contain retinal, a molecule that, upon exposure to light, changes shape. This change in shape sets off a chain of events that eventually causes the cell to send a signal to the brain. This signal, along with many others from other photoreceptor cells, is processed by the visual system of the eye and brain to create the image.

    Now, all of that above depends on the ability of the eye to focus light from one area of your vision onto a single spot on your retina. (So light from the center of your field of view is focused on the fovea, while light from 45 degrees off to the left is focused onto another part of the retina well off to the side from the fovea) In a healthy eye, the cornea does most of the focusing of the light and the lens simply provides fine adjustment to this focus. It's important to note that the image formed on the retina has little-to-no intrinsic information about distance. In other words, there's nothing your eye does to the light that allows you to determine distance to an object (other than the obvious job of focusing the light and allowing us to see in the first place). It's only by experience that the brain learns to accurately predict distances to objects, and it uses several methods to do so. A full list and their explanations can be found at the following wiki article: https://en.wikipedia.org/wiki/Depth_perception

    The 2nd part, how the visual system processes all this information, is beyond my ability to even begin to explain.

    The direction is determined by where the light is focused on the retina. The light from one falls on a different area of the retina than the light from the other. This results in an angular separation between the two. Basically, you can think of yourself placed at the center of a sphere. If one tube is directly in front of you, at zero degrees, then the other will be off to the side and you would have to turn yourself or your eye X degrees to bring that 2nd tube to the center of your field of view. That angle is the angular separation between the two.

    Now, obviously you and I don't go around thinking of things in terms of angles. In addition to the above, your brain does its best to find the distance to each object and then it combines the angle and the distance to determine which one is where. You yourself perceive all of this as one object being "about three feet to the left and two feet behind" the first.

    All of that is a simplified, mathematical, half-wrong explanation, but I think it gives you some idea of the basics. I doubt your brain inherently knows what angles are, and it isn't that great at measuring things at a distance either. Through experience it manages to figure out how to interpret the placement of the light on your retina in such a way as to allow you to see correctly. In addition, there are many, many ways in which the brain can be fooled, as any optical illusion will show you.
     
  8. Feb 21, 2016 #7

    sophiecentaur

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    EM waves of optical frequency are, by definition, 'visible'. The eye has no way of knowing if they come from a light source (LED / CFL / hot filament) or are reflected from an object. If the light gets in through the eye's lens then you can see it.
    Bear that in mind when you read descriptions of complicated experiments which can confuse one and shake ones faith in what one has learned in basic Science. All photons of a given frequency are identical and don't carry a flag to tell you their origin. That's a generalisation but it certainly applies to what this thread is discussing.
     
  9. Feb 21, 2016 #8
    Thanks a lot to you guys.

    I appreciate every answer of you and it definitely helped me to find a way of how I should classify my problem.
    I started to try to get to the bottom of it and obviously mixed up a lot of things which has led me to my mess.
    I again say thanks to all the posts and answers and if it was up to me the thread can be closed.

    Have a nice an brightened day.
     
  10. Feb 23, 2016 #9
    If you look at the night sky and it is a full moon you only see the reflected light from the moon ignoring stars .The area around the moon and the whole night sky is full of EM waves of optical frequency from the Sun but you don't see them they appear as dark surrounding the moon and planets..By definition they might be visible but in practice you don't see them unless reflected or refracted.
     
  11. Feb 24, 2016 #10

    sophiecentaur

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    I read this several times but couldn't decide what you meant. Then I wondered. Are you asking about seeing light that's not travelling towards your eyes? You cannot. If you see a laser beam, for instance, going across the sky in front of you, you are seeing light that's scattered by dust and water droplets and not the beam itself. Also air molecules scatter visible sunlight into your eye. Hence the sky is not black.
    There is no atmosphere round the moon so none of the light that heads away from your direction gets scattered towards you eyes.
    The moon can appear to have rings and halos around it. That's due to the light rays which come nearly in your direction and a small proportion arrives in your eyes off-axis.
     
  12. Feb 24, 2016 #11
    Yes that is the gist of it .Even when the sky is full of light beyond our atmosphere we can not see it unless it bounces of an object or goes through one and then travels to our eyes.
     
  13. Feb 24, 2016 #12

    sophiecentaur

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    The old adage is still true - even when you take GR into account; Light travels in straight lines.
     
  14. Feb 24, 2016 #13

    ZapperZ

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    I don't know. I just don't understand this starting premise.

    For example, look at one of those fluorescent light bulbs, or those colorful neon lights. What are you seeing? Aren't you seeing the light DIRECTLY from the source without it first being reflected off another object? If you are brave, and don't care about your eyes, then look directly at the sun!

    What am I missing here? {scratching head}

    Zz.
     
  15. Feb 24, 2016 #14

    sophiecentaur

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    I don't think you are missing anything. The OP was surprised that you can't see light as it travels past you and not directly into your eyes. (I think). It's just was well, of course, or we would be seeing everything at once in all directions. OWCH!
     
  16. Feb 25, 2016 #15
    You are seeing it being reflected or refracted of or through another object fluorescent neon and the sun all interact with the light which is produced.
    If you look carefully at a filament bulb it's the little element, fluorescent and neon it's the gas and the sun it's the elements that it is made of.
     
  17. Feb 25, 2016 #16

    ZapperZ

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    Say what?

    A discharge gas produces the exact wavelength of light that you see with your eyes. It isn't reflected off anything else. It is the GAS that is the source of such light. It isn't reflected off the gas! So you are seeing light directly from the source!

    Zz.
     
  18. Feb 25, 2016 #17
    Well not really it is usually contained within a glass tube and then there is the anode and cathode so there is all sorts of bouncing about the light does unless you can get the gas to discharge without any interference.
     
  19. Feb 25, 2016 #18

    davenn

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    yes, really !

    The Gas is ionised and is directly emitting light as a result as Zz said


    Dave
     
  20. Feb 25, 2016 #19
    So the filament bulb and the Sun is the same (that's around about where Zz came in)and there is no interference from other gases or objects.
    It would help if you could explain in more detail how this is possible.I have tried to explain in some detail how I imagine objects interfere with light could you show how the opposite can be true with gas ionisation and the equipment used to produce it.
     
  21. Feb 25, 2016 #20

    davenn

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    Zz already said that wasn't what was happening and I didn't say that either
    in this thread, the sun is just being treated as a source of light ( how the light is generated is not important)

    the same with a filament or fluorescent lamp
    there's slightly different processes going on in a filament compared to a fluoro tube

    in a filament lamp eg your home ceiling light or car headlamp, it's the filament that is directly emitting light, whether the globe is a vacuum or gas filled is irrelevant

    in a fluoro lamp the filaments at each end of the tube are NOT the source of the light. They are just providing a voltage potential across the gas filled tube
    The ballast/starter circuit allows for several high voltage spikes that are enough to produce an ionised path through the gas from one end of the tube to the other
    once that ionised path is established, then the lower mains voltage (240VAC in my country) is enough to maintain ionisation. The light is being emitted directly from the gas atoms in the tube


    of course this is all irrelevant to the OP Q of why we do or don't directly see EM waves
    That Q has already been well answered


    Dave
     
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