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A Shadow on the Wall

  • Thread starter dblader
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  • #1
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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
 

Answers and Replies

  • #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?
 
  • #3
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what do you mean by like triangles.
 
  • #4
LowlyPion
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what do you mean by like triangles.
Like triangles similar triangles, etc.

Won't the ratio of the sides be equal?
 
  • #5
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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.
 
  • #6
LowlyPion
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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.
So what is the distance of the light to the wall again?
 
  • #7
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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.
 
  • #8
LowlyPion
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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.
Similar triangles means the ratio of the complete side to complete side.

You were trying to compare a triangle to trapezoid.
 
  • #9
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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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