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Figuring out what image distance (lens and mirror) formula to use?

  1. Aug 5, 2012 #1
    I was practicing hw and I came across these two derived equations from 1/f=1/d0+1/di
    1st one 1/di=1/d0-1/f
    2nd one di=d0(f)/d0-f
    How do distinguish which one to use? and how did they get that derivation for the 2nd equation?
     
  2. jcsd
  3. Aug 5, 2012 #2

    Simon Bridge

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    I wouldn't worry about memorizing the different forms of the equations to use for which situation, focus on understanding the physics.

    Note - the second one is just what happens to the lensmakers formula if you solve it for di. The first one is incorrect.
     
  4. Aug 5, 2012 #3

    jtbell

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    1st one 1/di=1/d0-1/f

    Are you sure the right-hand side wasn't reversed, that is,

    $$\frac{1}{d_i} = \frac{1}{f} - \frac{1}{d_o}$$

    This is just a re-arrangement of your first equation. It's good algebra practice. First, get the 1/di all by itself on the left as in my equation above. Can you see where to go from there? (hint: how do you add or subtract fractions?)
     
  5. Aug 6, 2012 #4
    Oh yeah sorry oops I was just wondering about the 2nd one.
     
  6. Aug 6, 2012 #5

    Simon Bridge

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    No worries - did you have a go at the derivation yet? How did you get on?
     
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