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Uncertainty Calculation

  1. Jul 11, 2015 #1

    So I've stumbled upon this question asking us to calculate the uncertainty associated with the focal length.
    1/u + 1/v = 1/f
    u = 50mm with uncertainty 3mm
    v= 200mm with uncertainty 5mm

    F is calculated as 40mm, whats the uncertainty in this value?

    My first approach was to rearrange the formula to make f the subject, i.e. f= uv/u+v, then i applied the formula
    Δf/f = Δu/u + Δv/v +Δ(u+v)/(u+v)

    Substituting values,

    Δf/40 = 3/50 + 5/200 + 8/250
    Δf = 4.68

    However, the correct answer is 2.1. From the solutions, i understand that another approach to this question would be to calculate fmax and fmin and then add them up and divide them by two.

    Puzzled as to why my earlier steps doesn't yield the correct answer though, any help would be greatly appreciated!

    Thank you :-)
  2. jcsd
  3. Jul 11, 2015 #2
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