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1 question about light

  1. Sep 8, 2013 #1
    Hi
    Here is what confuses me about light .. if i am standing at a beach at new york assuming that the sky is perfectly cleary with no clouds and one friend of mine is sitting at a beach over the atlantic right across me if use a big source of light like a light projector will my friend be able to see me ?. Physic laws say he will as light cannot be stooped only delayed .. However i believe that the all the light rays will be seperating as the light travels futher . also doesnt the same thing happen to sound ?
     
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  3. Sep 8, 2013 #2

    Khashishi

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    When you say "over the atlantic" do you mean in Europe? Then no, because the Earth is curved, and the earth gets in the way.
     
  4. Sep 8, 2013 #3

    phinds

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    In addition to the Earth's curvature, "perfectly clear" sky means a sky still full of oxygen/nitrogen gas that does cause some dispersion over long distances.
     
  5. Sep 8, 2013 #4

    UltrafastPED

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    Even a very well collimated laser beam has a measurable divergence ... a typical light source spreads as r^2 ... the surface of a sphere centered on the light source.
     
  6. Sep 8, 2013 #5

    SteamKing

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    Do you and your friend regularly see people thousands of miles away?
     
  7. Sep 8, 2013 #6

    Drakkith

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    Yes, but physics also says that light can be absorbed, which is exactly what is happening over long distances. Dirt, dust, and even the air itself absorbs small amounts of the light as it travels. But the real kicker is that the curvature of the Earth prevents you from seeing your friend across the ocean. The ground absorbs light too.
     
  8. Sep 8, 2013 #7

    davenn

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    yeah but it has little to do with absorption sky or ground and mainly to do with light travelling in straight lines and not bending around the curvature of the earth ... as others have already said
    other than a little atmospheric absorption ... most of that beam of light will head out into space

    same with radio signals less than ~ 30MHz, except that the Ionosphere does a nice job of reflection/refraction and signals can be heard from over the horizon

    Dave
     
  9. Sep 9, 2013 #8

    meBigGuy

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    The light rays from a point source disperse as you described. The light rays from your body that are reflected directly toward the receiver do not further disperse in that same way. Not all the light from the projector is reflected directly toward you or he would be a mirror.

    If you were standing on the moon he could see you just fine assuming you could deal with atmospheric effects. Sunlight reflecting off the moon is no different than sunlight reflecting off you on the moon (well, white sand is a bit brighter).
     
  10. Sep 9, 2013 #9
    The moon has an albedo of about 0.12, more like coal or worn asphalt.
    It also reflects more in the direction of the incoming light, so it will be brighter with a full moon, if you stand in the center (as seen from the earth). You should still be a lot brighter than the moon with light-coloured clothes.
     
  11. Sep 9, 2013 #10
    Actually i don't think so .. Einstein has proved that light can be bent if it travels near a big mass like the sun , earth .
     
  12. Sep 9, 2013 #11
    Well we are both so small that cant be detected by the human eye so far . I am asking if light can travel so far and in my theoretical example i am thinking about a huge source of light .
     
  13. Sep 9, 2013 #12

    ZapperZ

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    You shouldn't simply read about something without understanding it, and then try to use it as a basis to support your argument. For example, just how much do you think light will be bent due to the earth's gravity? Can you see the skyline of Shanghai or Singapore from where you are in NY? After all, you think light can be bent by the earth's gravity?

    Understanding physics does not just mean having an idea of the concept. You must also understand the quantitative aspect of physics, before you try to use it. If you don't understand this aspect, then it safe to say that you shouldn't be using it too much as the foundation for your argument.

    Zz.
     
  14. Sep 9, 2013 #13
    Yes but i am only a High school student that just got accepted at University at the physics department . My knowledge's come from spending my free time in forums like this one or reading books .

    However , if its true (and i am not doubting you ) that's just a technical problem .. my real question is can light travel so far in a clear atmosphere of Earth ?
     
  15. Sep 9, 2013 #14

    Drakkith

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    It depends on the wavelength. For visible light, shorter wavelengths in the blue and violet region are scattered and absorbed much more easily than the longer wavelengths of the orange and red region. Still, if we could set up an infinitely long container of air and send light through it, all of the light would eventually be absorbed, no matter the wavelength.
     
  16. Sep 9, 2013 #15

    UltrafastPED

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  17. Sep 9, 2013 #16

    davenn

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    You can easily prove this to yourself. It only take ~40 - 50 km ( ~ 20 - 30 miles) if you are both at sea level, to loose line of sight with each other because of the curvature of the earth.
    So if you cant do it over that distance, how do you expect to be able to do it over the several 1000 km between USA and Europe ??

    My radio friends and I prove this all the time with our microwave radio link experiments
    We have to get up hilltops just to be able to extend that line of sight distance.

    Dave
     
  18. Sep 9, 2013 #17

    meBigGuy

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    There is nothing about the light rays or their radiation from the target object that limits the distance (as you inferred in the original post). They don't separate as the light travels. Are you clear on that?

    Light won't bend significantly due to the Earth's mass. You can read and calculate that bending. It will be a fun exercise.

    The transparency of the atmosphere is the only issue. You can search and read about atmospheric scattering, and about atmospheric clarity (smoke, moisture, etc).

    But your original hypothesis about the light rays separating is incorrect. My example with the a person standing on the moon should make that clear.

    Was that at all helpful?
     
  19. Sep 9, 2013 #18

    davenn

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    the rest of what you said was OK but the above statement ISNT the ONLY issue

    read my posts, the curvature of the earth IS THE ISSUE

    good grief guys, stop over thinking the problem ... Its not rocket science
    its purely a curvature of the earth/line of sight ( or lack of) problem

    attachment.php?attachmentid=61651&stc=1&d=1378770670.gif

    Dave
     

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  20. Sep 9, 2013 #19

    meBigGuy

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    He wasn't being all that literal about across the atlantic. In my opinion was interested in the effects of light through the atmosphere over a long distance and light propagation. I mentioned the curvature, but didn't consider it pertinent to the issues he was confused about. I also suggested he solve for the amount of light curvature due to the mass of the Earth. I'm surprised you got confused about the context of my "the only issue". You seem fixated on what YOU think is a big deal that you think no one understands. I suggest you re-read the OP and deal with his other misunderstandings.
     
  21. Sep 10, 2013 #20

    davenn

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    I suggest YOU reread his first and following posts ... its pretty obvious that he's referring to
    why they cant see each other or a strong light that's being shone towards the other person over a very long distance

    I stand by what I wrote earlier .... the OP doesnt understand light's straightline path. and he doesnt seem to comprehend how the curvature of the earth is the MAIN reason for not seeing eachother or the light from a long distance

    Lights on the earth's surface have no problem travelling through the atmosphere and into space and visa versa. Yes there is some absorption but not so much as it stops the light by any significant amount


    Dave
     
  22. Sep 10, 2013 #21

    CWatters

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    You aren't going to be able to see across the Atlantic even if the earth was flat.

    First there is a bit of a question over how you define "see" and "visibility". There are some definitions in the wiki entry that used in the aviation industry..

    http://en.wikipedia.org/wiki/Visibility

    Using their definition they say..

    Also worth a look at this article which has a lot in the subject of visibility...

    http://www.epa.gov/visibility/pdfs/introvis.pdf [Broken]

    One other problem is the ability of the human eye to resolve small objects. The eye couldn't see an object as small as a human at that distance even through a vacuum. The image in your eyeball would be smaller than a single rod/cone (I think).

    So lots of reasons why you couldn't see your friend across the Atlantic (or an equivalent distance in space).
     
    Last edited by a moderator: May 6, 2017
  23. Sep 10, 2013 #22

    cjl

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    If anything, I would say that's a fairly pessimistic estimate for clear-air visibility. In very dry desert environments, mountains 50 miles away can be perfectly visible, and I've seen them from 100+ miles a few times. Yes, it's uncommon for the air to be that clear, but it definitely happens.
     
    Last edited by a moderator: May 6, 2017
  24. Sep 10, 2013 #23

    meBigGuy

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    I have a perfectly clear photograph of Half Dome from Lick Observatory, and it is 120 miles away (and you can see the mountains beyond). I have seen it naked eye when flying in to San Jose airport, and could see much further. http://astronomy.snjr.net/blog/wp-c...semite_Valley_as_seen_from_Mount_Hamilton.jpg

    Not that this has anything to do with seeing 5,000 miles.

    Given particulate free air at sea level and some reasonable temperature, how much scattering occurs over 5000 miles?
     
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