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Ray diagrams, lenses and microscopes

  1. Jul 14, 2014 #1
    Hi all,

    I've been reading about biconvex lenses, drawing ray diagrams and real/virtual images. I'm now trying to translate this into how an image is formed in a compound infinity optics microscope. And this has raised some questions:
    -In an infinity optical system the object is placed at a distance (F) the focal point of the lens on the object side and therefore the lens converts the refracting rays to parallel ray bundles on the image side?
    -Because the object is always placed at the focal point an image is not formed by the objective?
    -The role of the tube lens is to converge the parallel rays travelling in line with the principal axis?
    -The point at which the rays converge on passing through the tube lens is the focal point of the tube lens on the image side, the intermediate image plane?
    -If the light travels in parallel lines between the objective and tube lens, is the light therefore collimated? And the objective is basically a collimator?

    Really i'm just wondering if my understanding on the above is correct? Ray diagrams tend to over-simplify things.

    Thanks for the help!
  2. jcsd
  3. Jul 16, 2014 #2

    Andy Resnick

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    Basically, you are on the right track. A few comments:

    1) don't confuse the focal length with the working distance. The working distance is the physical distance from the front of the objective to the sample plane.
    2) The 'infinity region', between the objective and the tube lens, allows auxiliary optical components to be inserted (filters, etc.) without requiring adjustment of the sample plane.
    3) By splitting the objective lens into objective+tube lens, aberration balancing and correction can be more easily designed (typically spherical and chromatic)- this is why infinity-corrected objectives made by one manufacturer can't easily be used with another manufacturer's microscopes.
    4) It's possible to use an infinity-corrected objective all by itself- when we say the image plane is 'at infinity', in practice that means 'a meter or so'. Of course, you lose the aberration correction of the tube lens.
  4. Jul 16, 2014 #3
    thanks andy that all makes sense. I had fairly simple lens and ray diagrams in my head so in this over-simplified example the focal length and the working distance are basically the same i think.. If you can create an image only with the objective lens then the light does eventually converge, so to think of it being parallel rays is only partially true i suppose.

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