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Physics Question

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  • 05-10-2009 6:08pm
    #1
    jumpguy
    Registered Users Posts: 7,962 ✭✭✭


    In the experiment to "Measure the Focal Lenght of a Converging Lens", in the graph to be drawn, is 1/u on the x axis and 1/v on the y axis? Please tell me that's the case, otherwise I've just wasted the last 30 minutes. Wondering so because I attempted to get the slope of my best fit line but didn't get the answer I was expecting.

    Is there any good way of telling which axis things go on instead of just learning it?


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    #2
    pathway33
    Registered Users Posts: 2,229 ✭✭✭pathway33


    http://www.youtube.com/watch?v=AoGDOT6U9pQ&feature=PlayList&p=9E9AC96BBD39720E&index=3


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    #3
    jumpguy
    Registered Users Posts: 7,962 ✭✭✭jumpguy


    pathway33 wrote: »
    http://www.youtube.com/watch?v=AoGDOT6U9pQ&feature=PlayList&p=9E9AC96BBD39720E&index=3
    Whew! Thanks.


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    #4
    Dirac
    Registered Users Posts: 22 ✭Dirac


    jumpguy wrote: »
    Is there any good way of telling which axis things go on instead of just learning it?

    The general rule for drawing ANY graph is that the variable which you have control over (the independent variable) goes on the x-axis and the other variable (the dependent one) goes on the y-axis.

    So, in the case of this experiment, you have control over the object distance (u), therefore 1/u goes on the x-axis. Consequently 1/v goes on the y-axis.

    Now write the equation for the focal length of a lens:
    1/f = 1/u + 1/v

    Re-write it so that it is in the form y = mx + c (the equation of a straight line), where y represents the term on the y-axis, i.e.

    1/v = -1/u + 1/f

    You should find that the slope is -1 and the intercept of the graph is 1/f from which you can calculate f.

    Have a look at the attached graph of 1/u vs 1/v for a convex lens of focal length 10cm. You'll see the equation has an intercept of 0.1 cm^-1, therefore f = 1/0.1 = 10cm.


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