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B Shadows in a circle

  1. May 10, 2018 #1
    Let us say we form a circle with stick, just like a sundial. But instead of one stick in the center, we put sticks all around on the circle. Will the shadows all point in the same direction?
     
  2. jcsd
  3. May 10, 2018 #2

    berkeman

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    What do you think is the answer, and why? And have you tried it?
     
  4. May 10, 2018 #3
    I have no way of trying it, and I would prefer to avoid presenting personal theories. I would like to have the "official answer" from established science.
     
  5. May 10, 2018 #4

    berkeman

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    That's admirable, but in this case we want to know what your thinking is on it. Try sketching it from several angles to try to get an idea.
    Is there no sunshine where you live?
     
  6. May 10, 2018 #5

    ZapperZ

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    What do you mean by you "... have no way of trying it..."? You do not have access to sunlight, and you can't scrape up a few sticks?

    Zz.
     
  7. May 10, 2018 #6
    let us say I am in jail and I am not allowed to have sticks.
     
  8. May 10, 2018 #7

    ZapperZ

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    Then, next time you get your "playtime" outside, ask a few of the other inmates to stand still, and look at their shadows.

    Zz.
     
  9. May 10, 2018 #8
    You are being unnecessarily rude. You know nothing of my personal circumstances and you do not need to know anything about them. There is something like privacy, maybe you have heard of it. You do not need to answer the question, but I certainly do not need to answer to yours about my personal life.
    If nobody is willing to answer this question then so be it.
     
  10. May 10, 2018 #9

    ZapperZ

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    And you are being unnecessarily presumptuous. I do not care about knowing about your personal life. What I was trying to do was to teach you how to fish, rather than just give you the fish! It was trying to get you to think and give you the ability to get the answer yourself, so that the next time you have a similar question, you will find the satisfaction in knowing that you are capable of thinking things through and arriving at an answer!

    But somehow, everyone nowadays simply wants to be spoonfed!

    Zz.
     
  11. May 10, 2018 #10
    The question is very simple. It has everything to do with the way light propagates. If it propagates in all directions, we should have shadows going in all directions, while if they all point in the same direction that would mean that the sun somehow functions like a directed spotlight. Out of respect for the forum rules I want to avoid any speculation and hear what science has to say on this simple matter. I do not need you to teach me fishing nor to spoonfeed me.

    << Post edited by Mentors to remove insults >>
     
    Last edited by a moderator: May 10, 2018
  12. May 10, 2018 #11

    Drakkith

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    A directed spotlight functions by focusing the light so that the rays emitted from any one point are very close to being parallel to each other. The Sun's great distance does the same thing. Light is emitted from the Sun in all directions, but rays from any one point on the Sun are extremely close to parallel to each other by the time they arrive on Earth, so the shadows all fall in the same direction.

    Note that the Sun is not a point-source, it is an extended source with the apparent shape of a disk, so there are an infinite number of different points on its surface. Light rays emitted from different points on the disk are not parallel to each other. They are only parallel to the light emitted by their emission point. Shadows created by the Sun are a little 'fuzzy' at the edges since light from part of the Sun may be blocked while light from another part may be visible. Imagine a Sunrise. You see the top of the Sun first, but the rest is occluded by the Earth. Until the Sun's disk is completely visible you occupy the fuzzy boundary of the immense shadow of the Earth. Or just edge around an obstacle slightly so that part of the Sun is visible to your eye and part is blocked (preferably do this near sunset/sunrise, else the glare will be too bright).

    The different regions of a shadow are known as the umbra, penumbra, and antumbra. See the following link for more: https://en.wikipedia.org/wiki/Umbra,_penumbra_and_antumbra
     
  13. May 10, 2018 #12

    jbriggs444

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    It propagates (ignoring diffraction, refraction, reflection and other sorts of scattering) in straight line paths from the source. If the source is in the east, one expects shadows to point westward, not eastward.
     
  14. May 10, 2018 #13
    I have trouble picturing this and I hope you will help further clarify the matter.
    First light goes in all directions, then it somehow meets at one location from which parallel rays are emitted?
    Replace the sticks with human observers looking (!) at the sun each from his position. They each see all of the sun. So, every point in space propagates the image of the whole sun?
     
  15. May 10, 2018 #14

    jbriggs444

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    The sun is that one location. The rays from the sun which reach the earth are the rays that were emitted in the direction of the earth. They are nearly parallel.

    The rays that go in all other directions do not reach the earth and are irrelevant to us.
     
  16. May 10, 2018 #15

    sophiecentaur

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    There was a chain of thought between those two conditions. Have you done any other thinking and reading about this, apart from moaning when the answer is not just dropped into your lap?
    If the Sun doesn't get as far as where you are, I imagine you may have some lighting or even a flash lamp / torch. Look at the shadows formed and try to connect that with possible paths of light from the source. Google Rectilinear Propagation and read about the early models for light. If you are in no position to do any experiments then Wiki will tell you all about light propagation and vision.
     
  17. May 10, 2018 #16
    If that is the case, how come we can see all of the sun? It is after all immense, in fact almost infinitely large relative to the retinal image.
     
  18. May 10, 2018 #17

    sophiecentaur

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    Google "Angle subtended by an object"
     
  19. May 10, 2018 #18

    fresh_42

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    It all depends on the location of the light source. Sunlight is from so far away, that it's rays can be considered parallel, in which case the shadows on such relatively small objects like a couple of sticks on a circle all point into the same direction - away from sun.

    If we had a lamp as a source of light and placed it in the center of the circle, then the shadows will be spread in all directions.

    In any case, we can draw a straight line from the light source to the object, and the shadow is a result of stopping such a line, here by the sticks.
     
  20. May 10, 2018 #19
    That is not an answer. I know what a subtended angle is. It would refer me back again to the explanations just given.
     
  21. May 10, 2018 #20

    jbriggs444

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    You see the bit of the sun on the left because that bit of the sun emitted some rays of light that happen to strike the pupil of your eye. You see the bit of the sun on the right because that bit of the sun also emitted rays of light that happen to strike the pupil of your eye. Those rays are almost parallel, but not quite. They illuminate (and damage) separate points on your retina. So please do not stare at the sun.
     
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