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Two lense system - Homemade spyglass.

  1. Dec 13, 2009 #1

    This is my first time posting here, so bare with me. I am from Denmark and I am at my final year of what corresponds to Senior High School. I am currently working on a project about "optical instruments" in physics (I was told that our A level corresponded to your applied physics level). I'ts the biggest project we do the entire year, so I am a little stressed about it. Anyway, sorry for the introduction.

    My question is regarding the path of light and the forming of a picture in a dobble lense system. Part of my assignment was to build my own spyglass. So the system in question consist of: One objective lense with a focal lenght of 30 cm and an ocular lense with a focal lengt of 5 cm. Both lenses have the same size. I know that, when looking at an object through the spyglass i have to ajust the objective lense in order to get a sharp image. But it is unclear to me, what exactlay happens with the lights path when i ajust the distance between the objective- and ocular lenses. I mean, in order to get a sharp image of an object at different distances i have to just the spyglass, but why is that?

    2. Relevant equations
    The magnification of the spyglass is given by f1/f2 = 30/5 = 6x times.
    (Not really sure if any equations is needed for me question)

    3. The attempt at a solution
    Not sure. Maybe it have something to do with the fact that, the longer the object is away to smaller the viewing angle gets. Atlest thats the only connection i can find.

    I guess, what i am basicly asking is: What exactly happens inside the spyglass when I adjust the lenght between the lenses in order to get a sharp image?

    Sorry for the long post, i hope you can bare with me. Also i appologize now, if this forum is not intendeed for these sort of questions. Feel free to ask anything, I can provide more information if needed. Thank you for taking your time.
    Last edited: Dec 13, 2009
  2. jcsd
  3. Dec 13, 2009 #2


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    Welcome to PF!

    There is a slight difference depending if you have a 30cm convex and a 5cm concave (as Galileo) or two convex (as Kepler)
    Either way it's easy to trace the rays with high school level optics (wiki geometrical optics)

    For an object at a very large distance (like a planet) then there is a simple correct location of the lenses which gives you a parallel beams out, that is an image at infinity for an object at infinity
  4. Dec 13, 2009 #3
    Wauv, thanks for quick answer and the welcome ;)

    Sorry about that - Both lenses are double-convex. (the radius of the sphere is not known)
    About the ray tracing - My question was more reagarding the theoretical explanation of the lights path through my spyglass. Actualy my assignment is to "1) Build a spyglass + 2) describe the way it work + 3) find its magnification".

    So I have done 1 and 3 (only theoreticaly tho). But, like I said, I am unsure what happens* when i adjust the spyglass in order to get a sharp i mage.

    *Sorry for the vague describtion. I mean what changes in the path of light, when i increase or decrease the distance between the two lenses, that provides the "sharp" image.

    I hope this cleared things up a bid :)
  5. Dec 13, 2009 #4


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    Assuming they are simple lenses (not achromats) you can easily work out the radius of curvature form the focal length - but thats not important.

    Tracing the ideal path through the system is easy - the lenses are just placed the sum of their focal lengths apart. But with spherical lenses in the real world there might be a position near this where some errors in the lenses cancel out. Without a computer ray tracing package and exact details of the lenses all you can do is move them and have some criterion for 'in focus'
  6. Dec 14, 2009 #5
    Right, thank you.
    I know that i can calculate the radius with the lensemakers formular, but like you said - not important :p

    But what is it that provides the sharp image? Why is it that when i look at on object close by (2 m) i need to increase the distance between the lenses (more than 35 cm) and when looking at object far way (20 m) i decrease the distance (less than 35 cm).

    When describing the functioning of the spyglass theorediacly, how should i describe that?
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