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Can you have an anti focal point of a lens?

  1. Jan 23, 2012 #1
    Can you have an "anti" focal point of a lens?


    Imagine putting a convergent lens in the path of a laser beam. I put a small light meter at the focal point of the lens.

    If I move the lens along the beam will there be positions of the lens where the light paths through it completely cancel at the focal point so that no photons are detected there?

  2. jcsd
  3. Jan 23, 2012 #2
    Re: Can you have an "anti" focal point of a lens?


    I've been thinking...

    In order to get the effect of light cancellation at a focal point maybe one needs to implant concentric rings inside the lens made of glass with a different refractive index. Adjacent rings could then be designed to cause the light to arrive at the focal point precisely out of phase.

    Maybe that would do it?
    Last edited: Jan 23, 2012
  4. Jan 23, 2012 #3
    Re: Can you have an "anti" focal point of a lens?

    If you could make a lens that focussed light to an "anti" focal point what happens if you place a thin absorbing sheet at that point?

    Would light somehow "tunnel" through the sheet as photons cannot be absorbed at the anti-focal point (zero probability for them to be detected there)?
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