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Science Forum

  1. 04 Jun '09 19:15 / 1 edit
    Found this link about a lens with a variable-focus that is achieved with no mechanical moving parts that relies on two liquids to refract the light and can very quickly adjust and maintain the focus of the lens using very little electric power.

    http://www.physorg.com/news308.html

    Although it technically has ‘no mechanical moving parts’ the liquids still have to move a bit for the lens to refocus.

    I am curious to know if it is possible to make an electrically adjustable-focus lens that doesn’t only have ‘no mechanical moving parts’ but literally has ‘no moving parts’ by making the lens out of a transparent material that when exposed to a change in electric field/current it instantly changes its refractive index?
    Does anyone know of such a material?
  2. Standard member sonhouse
    Fast and Curious
    09 Jun '09 21:01 / 1 edit
    Originally posted by Andrew Hamilton
    Found this link about a lens with a variable-focus that is achieved with no mechanical moving parts that relies on two liquids to refract the light and can very quickly adjust and maintain the focus of the lens using very little electric power.

    http://www.physorg.com/news308.html

    Although it technically has ‘no mechanical moving parts’ the liqu ...[text shortened]... field/current it instantly changes its refractive index?
    Does anyone know of such a material?
    The stuff we worked on for years at Lucent was Lithium Niobate. It is not a liquid, howver. But it does just that, apply an electric field and the refractive index changes. We used that technology to modulate laser beams at up to 10 gigahertz, putting 10 gigahertz signals on the laser beam. There are other ways now, with quantum physics, and the lithium niobate method is rather large, the chip we made was about the size and shape of a toothpick but the latest quantum physics version is more like the size of the width of a grain of rice. The fluids used for liquid lenses probably use the electric field to effect the surface tension. That's my guess anyway.
    One problem I see with such a system, is how do you get all the colors to focus at one point? If you use a single refractive index material, like the first lenses 400 years ago, they focused light alright and did a good job, they ground great lenses back then but there was a fundamental problem in that red would focus at a different distance than green or blue, each of those colors coming to focus at a different spot. It wasn't till the advent of doublet lenses with different refractive indexes that the problem was made manageable. Now there are very tricky solutions indeed, with graded refractive index lenses that do the job without having to invoke the old doublet idea. But I rather think these kind of lenses would suffer from the 400 year old telescope focus problem. It could be solved by having detectors sensitive to different colors at different distances, for instance, layered sensors, red going deepest, blue the shortest distance and green in the middle, something like that, but that would be a bandaid I would think.
  3. 10 Jun '09 11:09 / 1 edit
    Originally posted by sonhouse
    The stuff we worked on for years at Lucent was Lithium Niobate. It is not a liquid, howver. But it does just that, apply an electric field and the refractive index changes. We used that technology to modulate laser beams at up to 10 gigahertz, putting 10 gigahertz signals on the laser beam. There are other ways now, with quantum physics, and the lithium nio tance and green in the middle, something like that, but that would be a bandaid I would think.
    That is very interesting.

    I looked up Lithium niobate and found that it produces birefringence (or double refraction) that is proportional to the electric field it is exposed to (according to one website) thus it splits any ray of light into rays two rays of light when it refracts a ray of light.
    But wouldn’t that mean that a lens made of this stuff would cause TWO images at the back of a camera that have different focal distances from the lens thus making it impossible to produce a single clear well focused image at the back of the camera?
  4. 10 Jun '09 11:22 / 2 edits
    Originally posted by Andrew Hamilton
    That is very interesting.

    I looked up Lithium niobate and found that it produces birefringence (or double refraction) that is proportional to the electric field it is exposed to (according to one website) thus it splits any ray of light into rays two rays of light when it refracts a ray of light.
    But wouldn’t that mean that a lens made of this s ...[text shortened]... hus making it impossible to produce a single clear well focused image at the back of the camera?
    Hang on; I just read at: http://en.wikipedia.org/wiki/Lithium_niobate
    That:

    “…Lithium niobate has negative UNIAXIAL birefringence which depends slightly on the stoichiometry of the crystal and on temperature
    …” (my emphasis)

    So I looked up “UNIAXIAL” and got:

    “…Having one direction along which double refraction of light does not take place…”

    So this appears to answer my question -the answer being that ALL the light of a light ray of a given wavelength is NOT split into two but ALL of it is diffracted into the SAME direction.
    But now I am confused; if there is “one direction along which double refraction of light does not take place” then in what sense is it “double refraction”!!!? -this appears to be a logical contradiction to me that makes me wonder if I have somehow completely misunderstood something here.

    And what does the “negative” mean in the term “negative UNIAXIAL birefringence”?

    I would also like to know what kind of voltage would be needed to change the refractive index of a refocusable lens made of Lithium niobate enough to have useful amount of refocusing? -I mean, are we talking about less than one volt or hundreds of volts or what?
    And is there any good technical reason why refocusable lenses made of Lithium niobate are not used much (are any used at all?) other than problems with fabrication and manufacturing cost?
  5. 11 Jun '09 10:16
    A way to have a mirror with a variable-focus that is achieved is to put (liquid) mercury into a bowl, and start to spin it slowly, and its surface turns into a perfect parabolic shape. So by adjusting its rotation you can have any focal length you want.
  6. Subscriber AThousandYoung
    Do ya think?
    15 Jun '09 06:35
    Originally posted by FabianFnas
    A way to have a mirror with a variable-focus that is achieved is to put (liquid) mercury into a bowl, and start to spin it slowly, and its surface turns into a perfect parabolic shape. So by adjusting its rotation you can have any focal length you want.
    as long as its horizontal
  7. 15 Jun '09 06:44
    Originally posted by AThousandYoung
    as long as its horizontal
    Yes, as long it is horizontal. No problem.
    The sun-observatories uses fixed mirrors at the bottom, and variable mirrors in the top of their observatiories. In the between they have thir fixed lenses.