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A Shadow on the Wall

  1. Mar 5, 2009 #1
    1. A rectangular piece of cardboard 21.7cm high is held 1.0m in front of a point source of light. The cardboard is 2.6m away from a wall. How tall is the shadow on the wall?

    I have no idea how to do this my prof never talked about this in class and I looked threw the book with no help. I emailed my prof and he told me to google it. The way I thought to do it was to use the b/a=c/d=e/f , and use the ratio of 2.6m/1m = d/.217m and i got d=.56m but it wasn't correct. if any of you guys know how to do this I would greatly appreciate it.

    Thanks,

    Dblade
     
  2. jcsd
  3. Mar 5, 2009 #2

    LowlyPion

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    Welcome to PF.

    Just make a drawing. Your light is a point source. Won't then the projected dimensions be able to be determined by like triangles?
     
  4. Mar 5, 2009 #3
    what do you mean by like triangles.
     
  5. Mar 5, 2009 #4

    LowlyPion

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    Like triangles similar triangles, etc.

    Won't the ratio of the sides be equal?
     
  6. Mar 5, 2009 #5
    im pretty sure that they would be equal. a=distance from source to object b=distance to wall c=height of object d= height of object on wall so i plugged in 2.6m/1m = d/.217m so i got d=.564m i plugged that in and it said i was wrong.
     
  7. Mar 5, 2009 #6

    LowlyPion

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    So what is the distance of the light to the wall again?
     
  8. Mar 5, 2009 #7
    i figured it out. I took 1m+2.6m=3.6m*.217m=.781m which is the correct answer. but can you explain why that is i know how to do it know but i need to understand it. thanks for your help.
     
  9. Mar 5, 2009 #8

    LowlyPion

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    Similar triangles means the ratio of the complete side to complete side.

    You were trying to compare a triangle to trapezoid.
     
  10. Mar 5, 2009 #9
    okay so d is the whole side so when i was multiplying 2.6m it wasn't the whole side of the triangle but 3.6m is the whole side of the triangle. thanks
     
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