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Two foci on rectilinear congruence of light rays

  1. Dec 7, 2014 #1
    At page 135 of Max Born's "Principles of Optics", he proves the statement that there are two foci on each ray of a rectilinear congruence.

    Here a rectilinear congruence of light rays is defined as a collection of straight light rays such that for each point in space there is one ray of the congruence going through that point.
    A foci is defined as a point on a specific ray such that the distance to a neighbouring ray vanish to first order.

    Question: Intuitively, I would think that straight rays would just converge at a single point, not two. How can it be two places on a single straight ray where it get "close" to its straight neighbouring rays?
  2. jcsd
  3. Dec 12, 2014 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
  4. Dec 12, 2014 #3


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    Perhaps he means that there are two points where the terms vanish to first order, right before and right after the point of intersection?
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