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Experimentally finding focal length of small biconvex lens?

  1. Dec 12, 2016 #1
    I have fabricated a few biconvex lens (12mm diameter) for an experiment and am trying to find their focal lengths. My though is to set up three parallel lasers and find where the beams converge. However, even with the smallest laser diodes I could find, three beams surpass the diameter of the lens. Would two beams be sufficient? Or does anyone have an alternative method of experimentally finding the focal length of small biconvex lens?
  2. jcsd
  3. Dec 12, 2016 #2


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    Science Advisor

    1) Take the lens and a piece of white paper out in direct sunlight.
    2) Focus the Sun on the paper by varying the lens-to-paper distance.
    3) When in focus, the distance from the lens to the paper is the focal length.
  4. Dec 12, 2016 #3


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    Well done fabricating your lens - that is no mean feat. (I do this for a living - contact lenses are concave/convex lenses about 14 mm in diameter).

    What radius did you intend to make back and front? And what is it made of, glass or acrylic/perspex?
  5. Dec 13, 2016 #4
    Would this work with a far away light source indoors, like a bright lamp across the room? There's a liquid component to my lens and I don't want it to freeze outside.
  6. Dec 13, 2016 #5
    Should be around -3 diopters. I made them out of acrylic (CNC'ed then heavily polished).
  7. Dec 13, 2016 #6


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    Yeah, that should work. You can calculate the error introduced by the finite distance to the light source to see if it's acceptable to you.
  8. Dec 13, 2016 #7


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    If you are working in dioptres then your friendly optician/optometrist will be have exactly the right instrument to measure the power of your lens. They will also be able to assess the quality of the focus.

    (If your lens is bi-convex it will have a plus power BTW).

    Well done polishing it too - it is a tricky process and there is definitely an art to it.
  9. Dec 14, 2016 #8


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    As long as your source distance is greater then 10x the focal length, a direct measurement will be as good as your ability to measure.
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