Shadow Height on Wall: 2.6m Light Source

[dblader]

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

 

[LowlyPion]

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?

 

[dblader]

what do you mean by like triangles.

 

[LowlyPion]

what do you mean by like triangles.

Like triangles similar triangles, etc.

Won't the ratio of the sides be equal?

 

[dblader]

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.

 

[LowlyPion]

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?

 

[dblader]

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.

 

[LowlyPion]

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.

 

[dblader]

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