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Producing umbra and penumbra with light sources

  1. Jun 19, 2005 #1
    when two souces are used, how many sources must be blocked by the object to produce the unbra? and the penumbra?

    I looked it up in my text book and on the internet and i still can't find the answer.
     
  2. jcsd
  3. Jun 19, 2005 #2
    help please

    Can someone help me please
     
  4. Jun 19, 2005 #3

    HallsofIvy

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    Both have to be blocked for the "umbra", only one for the "penumbra".
     
  5. Jun 19, 2005 #4

    Hurkyl

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    Do you know the definitions of umbra and penumbra? The answers should have been obvious. Even doing something as crude as punching

    "define umbra"
    "define penumbra"

    into google and looking at the web definitions gives us phrases like:

    For penumbra:
    "Literally partial shadow; the partially lighted area around any completely darkened area (umbra) of full shadow."
    "The shadow that results when only part of the bright object is occulted"
    "region of partial shadow that is outside the umbra; the light source is partially blocked."

    For umbra:
    "The area of total darkness in the shadow caused by an eclipse."
    "The umbra is the area of total shadow (compare to penumbra)."
    "The portion of a shadow that receives no direct light from the light source."
     
  6. Jun 19, 2005 #5
    thank you so much
     
  7. Jun 19, 2005 #6
    I need help with shadows

    When three sources are used, how many sources must be blocked by the object to produce the umbra? the dark penumbra? and the light penumbra?

    Could anybody help me please.
     
  8. Jun 19, 2005 #7

    Hurkyl

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    Have you tried at all to figure it out yourself?
     
  9. Jun 19, 2005 #8
    yes i have tried to figure it out but i can't figure it out at all
     
  10. Jun 19, 2005 #9

    Hurkyl

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    In your own words, what are the definitions of "umbra", "dark penumbra", and "light penumbra"?

    Can you at least identify the conditions on being in the umbra or penumbra?
     
    Last edited: Jun 19, 2005
  11. Jun 19, 2005 #10
    The umbra is part of the shadow that is entirely dark. i don't know the definitions of dark penumbra or light penumbra.
     
  12. Jun 19, 2005 #11

    Hurkyl

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    Ok, that's good. Can you describe the penumbra? Do you have a guess as to what "light penumbra" and "dark penumbra" mean? I think that "light" and "dark" weren't being used as technical terms here, just descriptive.
     
  13. Jun 19, 2005 #12
    so i would put a part where some light falls.
     
  14. Jun 19, 2005 #13

    Hurkyl

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    I don't understand what you've suggested.
     
  15. Jun 19, 2005 #14
    When three sources are used, how many sources must be blocked by the object to produce the umbra? the dark penumbra? and the light penumbra?
     
  16. Jun 19, 2005 #15

    Hurkyl

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    Didn't you already figure one of those questions out?
     
  17. Jun 19, 2005 #16
    i don't know
     
  18. Jun 19, 2005 #17

    OlderDan

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    Draw yourself some diagrams. If you have one light source blocked by an object, there will be a shadow that has an umbra and a penumbra. If the shadow falls on a screen, you can draw the umbra and penumbra areas that hit the screen. I will assume you know what those are. If you have two light sources, each source will have its own umbra region and penumbra region behind the object. On a screen, there may or may not be some umbra overlap, some region where the umbra of one shadow overlays the penumbra of a second shadow, and some regions where only penumbra overlap. There could be no overlap at all if the screen is too far away from the object, but you can move the screen closer to get all the different kinds of overlap that are possible. With three sources you can have more combinations of overlap contitions including: three umbra overlap, two umbra and one penumbra, one umbra and two penumbra, three penumbra. Try to sketch these various possibilities as they might appear on a screen and then answer the question.
     
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