Shadow Height on Wall: 2.6m Light Source

In summary, to determine the height of a shadow on a wall, a drawing must be made and the ratio of the sides of similar triangles must be used. In this case, the distance from the light source to the object is 1m and the distance from the object to the wall is 2.6m. By adding these two distances (1m+2.6m=3.6m) and multiplying by the height of the object (0.217m), the height of the shadow can be determined (3.6m*0.217m=0.781m). This method works because the ratio of the sides of similar triangles is equal.
  • #1
dblader
5
0
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
 
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  • #2
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
what do you mean by like triangles.
 
  • #4
dblader said:
what do you mean by like triangles.

Like triangles similar triangles, etc.

Won't the ratio of the sides be equal?
 
  • #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.
 
  • #6
dblader said:
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
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
dblader said:
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
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
 

Related to Shadow Height on Wall: 2.6m Light Source

What is the "Shadow Height on Wall"?

The "Shadow Height on Wall" refers to the length of the shadow cast by an object on a vertical surface, such as a wall, when a light source is shone on it.

What does a "2.6m Light Source" mean?

A "2.6m Light Source" refers to the distance between the light source and the object casting the shadow. In this case, it means that the light source is 2.6 meters away from the object.

How is the shadow height on a wall determined?

The shadow height on a wall is determined by the angle at which the light hits the object and the distance between the light source and the object. The longer the distance between the light source and the object, the longer the shadow will be.

What factors can affect the shadow height on a wall?

The factors that can affect the shadow height on a wall include the distance between the light source and the object, the angle at which the light hits the object, and the size and shape of the object casting the shadow.

Why is knowing the shadow height on a wall important?

Knowing the shadow height on a wall can provide valuable information about the position and orientation of the light source, as well as the size and shape of the object casting the shadow. This information can be useful in various fields such as architecture, astronomy, and photography.

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