Silly lens question

Discussion in 'Miscellaneous Equipment' started by jjstafford, Jun 2, 2005.

  1. jjstafford

    jjstafford Inactive

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    Suppose you have two lenses; one is made of two parts cemented together and the other is of the same overall dimesions, but of one piece as shown below. Further, let's say the former is bound by a conventional cement.

    The question is: why do the two examples behave differently? Or do they? Is it the break made by the cement? If it is by the cement, then what if the cement had the same diffraction qualities of the glass?

    Or is something else happening, for example the light rays entering the first lens "have knowlege" of its boundaries an reconform to act differently when they hit the second?
     
  2. wfwhitaker

    wfwhitaker Member

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    In the two-piece example the elements are made of different types of glass possessing different optical properties.
     
  3. jjstafford

    jjstafford Inactive

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    And if they were of the same glass?
     
  4. BradS

    BradS Subscriber

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    One would be easier and probably cost less to manufacture.
     
  5. wfwhitaker

    wfwhitaker Member

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    Beats me. I'm not an optical engineer, nor do I play one on TV. My best guess is that two cemented elements of the same identical glass would behave as a simple lens. But maybe one more knowledgeable than I will reply.
     
  6. rbarker

    rbarker Member

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    I'm not an optical engineer, either, but Will's guess, given the (potentially artificial) parameters here, certainly sounds logical.
     
  7. Dan Fromm

    Dan Fromm Member

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    Actually, that's how Zeiss made center filters. Two pieces of glass of the same refractive index and dispersion, one plano-convex, the other plano-concave. The plano-convex one is thicker in the middle, is made of dark glass. The plano-concave is made of clear. Presto! Center filter.

    John, you have a history of asking stupid-seeming questions in order to be given the solution you want to a problem. What are you trying to accomplish this time?

    Cheers,

    Dan
     
  8. George Papantoniou

    George Papantoniou Member

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    "Actually, that's how Zeiss made center filters."

    Funny, but I always wondered how they made them. I fantasized a guy with an airbrush (or spray can) with gray paint pointing it at the centrer of a round piece of glass... :smile:
    Now I know... But how do they make them so thin ? If they're made of two pieces of (not flat) glass, they should be thicker, shouldn't they ?

    Thank you, Dan.
     
  9. jjstafford

    jjstafford Inactive

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    Gosh, am I so transparent? Really, I'm simply curious about just this one point which I thought would be simple enough not to warrant spending all weekend in the library to figure out. Too much time on my hands right now; perhaps I'd best hit the books.
     
  10. Ole

    Ole Moderator Staff Member

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    If two clear pieces of glass with the same refractive index and dispersion are cemented together, the only difference over a single piece of glass would be more flare.

    At least that's my best guess...
     
  11. MattKing

    MattKing Subscriber

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    It occurs to me that the two pieces might be less expensive to manufacture
     
  12. glbeas

    glbeas Member

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    Not likely, there would be four surfaces to grind instead of two.