Focal length of converging thin lens graphs

  • #1
jribs
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Homework Statement:
So in my physics assignment we had to graph the negative of magnification (-M) versus distance between image and lens (q). From this we had to find the focal lengths from the intercepts. I have the focal length using the x intercept of the equation M=1-q/f (which I rearragened to -M= q/f -1) but I can’t figure out how to find the focal length using the y intercept.
Relevant Equations:
Y=mx+b is equivalent to -M=1/f•q -1
Focal point using x intercept: x0=-b/m=f
I tried -M=pq = f/(f-p) = f/f - f/p = 1-(f/p)
=> M+1= -f/p
=> -Mp-p=f

But since it’s the y intercept, p will equal zero and so the focal length will be 0?
 

Answers and Replies

  • #2
kuruman
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I tried -M=pq = f/(f-p) = f/f - f/p = 1-(f/p)
=> M+1= -f/p
=> -Mp-p=f

But since it’s the y intercept, p will equal zero and so the focal length will be 0?
If your task is to graph the negative of the magnification -M versus the distance of the image from the lens q, what is p doing in your equations? Starting from the thin lens equation and the magnification equation, eliminate the object distance p to get an equation between M and q. Then substitute y for (-M) and x for q and see if you can bring the result to the form y = a x + b.
 

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