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Homework Help: Focal length calculation and graph Q

  1. Jun 7, 2007 #1
    Hi folks, just joined the forums :smile:
    Hope you dont mind me firing a few questions now and again, i have some reports to do over the weekend which is the last of my HNC in Chemical Engineering :!!)
    A lot of it is basic physics which iv done in the past but its just the odd bit that catches me out.

    So anyway, iv done an expeiment with a converging lens where you alter the object distance and image distance ( U & V ) to get focused images on the screen.
    With these results iv plotted a graph of V against U which produces a curved slope (diagram 1)
    u and v.JPG
    Then i done a graph of 1/V and 1/U which produces a straight line which intercepts the x and y axis, these values being 1/f (diagram 2)
    My question is

    Why is the graph of U and V not useful for finding the focal length of the lens accurately?

    probably an easy question but i just cant put my finger on it :shy:

    cheers :smile:
  2. jcsd
  3. Jun 7, 2007 #2
    use similar triangles to proove that the formula for a lens is (1/U + 1/V = 1/f)
    or u can find it anywhere on the internet. it is a very easy proof, won't take you 5 min.

    therefore it is 1/U and 1/V not U and V
  4. Jun 7, 2007 #3
    not entirly sure what you mean, triangles?
    anyway, that 1/V vs 1/U graph with my experimental results is proof that the equation is true.
    im just asked why the U and V graph doesnt give an accurate value for the focal length?
  5. Jun 7, 2007 #4

    if you draw your object, lens and image on a piece of paper with two of the rays you use to draw a ray diagram, you can see that there are some similar triangles on that figure. Try using similar triangles and getting two equations involving U, V and f.

    then solve them simultaneously to get "1/U + 1/V = 1/f "

    (if you cannot I can give you the solution but try it first :)

    U and V graph cannot give you an accurate value for the focal length because the relation between the two is not related to f in any simple way.
    if u play around with the equation I gave you above you might get " "[(v+U)/U]*f=V" but this won't make it any easier.

    perhaps the answer to your question is that the nature of lenses does not provide us with that simple relations where you plot U versus V and get a slope or a y intercept of f
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