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Optics - Angular Magnification

  1. Oct 5, 2012 #1
    1. The problem statement, all variables and given/known data
    The eyepiece of a compound microscope has a focal length of 2.50 cm and the objective has a focal length of 1.5 cm. The two lenses are separated by 17 cm. The microscope is used by a person with normal eyes (near point at 25 cm). What is the angular magnification of the microscope?

    a) 283 x
    b) 145 x
    c) 97 x
    d) 113 x
    e) 242 x


    2. Relevant equations
    Angular Magnification = Near point / f (eye piece) = 25cm / f (eye piece)

    Total magnification = 25cm(s1') / (f1f2)



    3. The attempt at a solution
    At first, I just thought it was asking for total magnification and got 113x, which is wrong. I know that the angular magnification is 25 / f (eye piece) so 25/2.5? Clearly not right. Perhaps I need to use the object-image formula (1/s + 1/s' = 1/f) to derive more information, but I'm not sure how. Help would be appreciated =)
     
  2. jcsd
  3. Oct 5, 2012 #2

    Simon Bridge

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    Why did you use 25cm for the near-point in the formula?
    How may the other two measurements be used?

    Do you know how the total magnification is related to the angular magnification?
     
  4. Oct 6, 2012 #3
    I used 25cm because that is the normal near point, and it says it in the problem statement. The total magnification = angular magnification * lateral magnification, where lateral magnification = s1'/s1
    Since I know that the total magnification is 113.333 (as calculated in my original post), I just have to divide by the lateral magnification to get the angular magnification. I don't know which measurements to use for lateral magnification though...
     
  5. Oct 6, 2012 #4

    Simon Bridge

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    <checks> Oh so it does - well done ;)
    How would you normally compute the lateral magnification in a system of two lenses?
     
  6. Oct 6, 2012 #5
    Using the object-image relation formula (1/s + 1/s' = 1/f), the image from the first lens becomes the object of the second lens. I tried doing that, but I'm not sure which lens acts as the object, and would the first object distance be 17cm?
     
  7. Oct 6, 2012 #6

    Simon Bridge

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    You are doing well - see how these questions refine the problem by stages?
    Which lens gets the light from the actual object first?
     
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