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Lens - What physical features makes a lens thin?

  1. Dec 19, 2012 #1
    lens -- What physical features makes a lens thin?

    What physical features makes a lens thin? How can you differentiate one lens from the other based on their thickness? What is the measure of their thickness?
     
  2. jcsd
  3. Dec 20, 2012 #2

    sophiecentaur

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    Re: lens

    Are you referring to the fact that you have to pay through the nose for posh spectacles?
    The focal length of a lens depends upon the curvature AND the refractive index of the glass / plastic. You need less curvature (hence you can have a thinner lens) if you use a higher refractive index.
    The thinner the lens, the less distortion (spherical aberration) you get because the curvature is less.
    I have a friend who had the option of thick 'bottle glass' specs, which would have a very limited field of undistorted view, with rings round the outside or some reasonable ones which were thinner and were made of glass with a vast amount of heavy metals in the mix (£££££££!). With the expensive ones, her eyes look 'normal' and she can see over a much bigger range of angles.
     
  4. Dec 20, 2012 #3
    Re: lens

    For a given lens power, the lens made of higher index material can be thinner. The thinner lens should have lower geometric aberration. However, a lot of higher index materials have higher chromatic aberration (lower Abbe number), so it's a tradeoff. My experience with eyeglass stores is that they want to push you to buy expensive lenses, which often have low Abbe number, so they don't tell you about this tradeoff.

    You can measure thickness with calipers.
     
  5. Dec 20, 2012 #4

    sophiecentaur

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    Re: lens -- What physical features makes a lens thin?

    Yebbut the thin ones are a lot lighter.
     
  6. Dec 21, 2012 #5
    Re: lens -- What physical features makes a lens thin?

    In general, If you want to minimize spherical aberration, we should have less radius of curvature or more radius of curvature?
     
  7. Dec 21, 2012 #6

    sophiecentaur

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    Re: lens -- What physical features makes a lens thin?

    Bigger radius - that would, I think, be describes as having less curvature. Nicely confusing language! Zero curvature (infinite radius) would have no spherical aberration - but not be much use as a lens, of course.
     
  8. Dec 21, 2012 #7
    Re: lens -- What physical features makes a lens thin?

    If it is having infinite radius, it will be a glass slab:).
    Is there any relationship between parabollic mirrors and spherical mirrors?
     
    Last edited: Dec 22, 2012
  9. Dec 22, 2012 #8

    Drakkith

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    Re: lens -- What physical features makes a lens thin?

    Uhh, one is curved in the shape of part of a sphere, the other is curved in the shape of a paraboloid. The latter will not have any spherical aberration, but will have severe coma when your light isn't right on the optical axis.
     
  10. Dec 22, 2012 #9
    Re: lens -- What physical features makes a lens thin?

    Thank you all.
     
  11. Dec 22, 2012 #10

    sophiecentaur

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    Re: lens -- What physical features makes a lens thin?

    There is not much difference for a small diameter, large radius reflector of either type but the essence of the paraboloid is that all rays parallel with the principle axis are reflected exactly to one focus. All paths for rays reflected onto the focus are exactly the same length. For a small field of view this gives a 'perfect image'. Afaik, though, reflecting telescope mirrors are ground to be spherical because that's what you get when you grind one flat plate against another, with a circular 'rolling' motion - one convex and one concave spherical surface. They always use a long focal length so spherical aberration is slight for any shape. Microwave dishes are normally (always?) parabolic because they have a relatively wide aperture for their focal length and you get a good image onto the feed point for sources 'on axis'. They aren't always 'symmetrical', though but have an offset feed.
     
  12. Dec 22, 2012 #11

    sophiecentaur

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    Re: lens -- What physical features makes a lens thin?

    Ah well - I now see that reflectors are usually paraboloidal. I guess the ones I was referring to were the home constructed ones that my colleagues made.
     
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