1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    Homework Helper

    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

    User Avatar
    Homework Helper

    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

    User Avatar
    Homework Helper

    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

    User Avatar
    Homework Helper

    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: A Shadow on the Wall
Loading...