Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Optics question

  1. Apr 5, 2004 #1
    I am trying to find F, i have L = 60cm and U = 31cm, how would i calculate f (focal length)???

    and also how would i find the uncertainties?

    thank you very much
  2. jcsd
  3. Apr 5, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Not sure about the uncertainties but I assume U and L are object and image distances so the formula you need is

    [tex]\frac{1}{L} + \frac{1}{U} = \frac{1}{F}[/tex]
  4. Apr 5, 2004 #3
    I don't what else you were given to work with but the general rule for calculating uncertainties is

    |df/dx| * Uncert. in x + |df/dy| * Uncert. in y

    In your case, x could be L and y could be U. The derivatives would be partial deriv. of course. If you're not familar with partial derivatives then you can do it the hard way. Make both L & U smaller by their uncertainties and calc. f. Then make them both larger by the same amounts and recalc. f. The differences between these two f values and the one calc. with the given U & L values would be the uncertainties.

    If you are not given the uncertanties in U & L then you have to estimate them. If they are measured, you can estimate the uncertainty as 1/2 of the smallest division on the ruler. If not, you can use the significant figures in their values as a guesstimate. 30 cm +/- 1 cm for example.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook