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General Optics Question

  1. Sep 28, 2015 #1
    1. The problem statement, all variables and given/known data
    I have a compound thick lens problem that I need to find the sign of the refractive power.

    2. Relevant equations
    Thick lens equation and focal length equation.

    3. The attempt at a solution
    I believe all I need to do is calculate the focal length of the first two thick lenses and then use that result as the first "lens" in the same focal length equation again:

    ##\frac{1}{f_{12}}=\frac{1}{f_{1}}+\frac{1}{f_{2}}-\frac{d_{12}}{f_{1}f_{2}}##
    ##\frac{1}{f_{123}}=\frac{1}{f_{12}}+\frac{1}{f_{3}}-\frac{d_{123}}{f_{12}f_{3}}##

    Is this a correct (or correct enough approximation) to get the sign?
     
  2. jcsd
  3. Sep 29, 2015 #2

    andrevdh

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    Maybe you could "send in" parallel rays (object at infinity) and try and work out if the image formed by the system is real (+ sign) or virtual (- sign)? What information about the system is available?
     
  4. Sep 29, 2015 #3
    I have all the dimensions of the lenses and distances between in the system.
     
  5. Sep 29, 2015 #4

    andrevdh

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    HyperPhysics have decent summary of the relevant theory - Gullstrand's equation and principal planes.
     
  6. Sep 29, 2015 #5
    What 'd' should be used for calculating the power when including the third lens? So I have the first two lenses easy enough. But I'm not sure if I take the P12 to be the distance of the second lens, the average of the distance from 1->3 and 1->2 or just the total distance. I suspect it is the distance from the second lens but I"m not 100% sure.
     
  7. Sep 29, 2015 #6

    andrevdh

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    Maybe you should treat each lens separately, otherwise you run into the problem you are now facing - how to locate the second principal plane, which in this instance seems to be "impossible"? Sorry not my forte.
     
  8. Sep 29, 2015 #7
    Yeah, its been several years since I've had any optics classes so I'm struggling to remember. I recall doing some matrix multiplication for compound lenses but I don't dare delve that deeply. I don't think it is impossible, just more effort.
     
  9. Sep 29, 2015 #8

    andrevdh

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