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Homework Help: Help with A-level physics coursework. focal lengths of lenses

  1. Nov 1, 2003 #1

    im in a bit of a panic at the moment

    i have 2 days to complete my physics coursework on focal lengths.

    the title is:
    "you are given two convergin lenses of different lengths. you are to devise an experiment:-
    -to measure the focal length of each lense
    -to measure the focal length of the lens combination
    -to try to find any relationship between the individual focal lengths and the combined focal length."

    ive got my readings and some graphs, but there is a little problem. when the graph is plotted, should the line dip back upwards when the image distance gets close to zero? if so why does it do this? if this isnt supposed to happen, and it is because of my crappy reading taking, then i think that i could be up a certain creak, without a certain paddle because i wont have the chance to re-take them...

    also, from these graphs how do i calculate the focal lengths? and then the link between the combined and sepourate focal lengths?

    any other useful info you think imight need for this assignment would be great too! :smile:

    thanks a lot for any help!!

    Last edited by a moderator: Nov 1, 2003
  2. jcsd
  3. Nov 1, 2003 #2
    p.s. i just read the "please read before posting" thread. i dont want this coursework done for me, i just need a little help in some areas :)

    sorry if i put this thread in the wrong place

  4. Nov 1, 2003 #3
    What graphs have you plotted? That would help us..

    Also have you used the equation:

    1/v = 1/u + 1/f (where distances from the lens towards the object are negative and distances from the other side of the lens are positive)
  5. Nov 1, 2003 #4
    well, i have plotted image distance, y, against object distace, x.

    the line on the first lens starts at 125cm at 100cm object distance, and then curves down to 78.5cm image dist against 30cm object. but THEN at 20cm object dist the image distance gets larger again and goes all the way up to 85cm. is that normal?


    i was goven the equation:

    1/f = 1/v + 1/u yeah. but when getting the values for u and v do i take numbers off the best fit line on the graph or what??

  6. Nov 1, 2003 #5
    Look at your assignment and ask yourself why you have plotted "image distance, y, against object distance, x." What did you hope it would show?

    Use the equation given and do some calculations.

    I would also suggest that if this assignment is important to you, perhaps you should research the topic of lenses and try to apply what you find out.
  7. Nov 1, 2003 #6
    thanks a lot for the help!

    what graphs should i be looking at then?

    and the calculations? do i just pick out some of the values for v and u that i have recorded and work with them?

    really appreciate the help!

  8. Nov 2, 2003 #7
    A graph is not always essential....

    Yes, use some u and v values and get calculating. You can easily find the focal length from these values and the more you have the better result you can get by taking an average (which is in effect what you do with a 'best fit' line on a graph)
  9. Nov 3, 2003 #8
    thanks very much :wink:

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