Physical meaning of directrix?

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SUMMARY

The discussion centers on the physical significance of the directrix in conic sections, particularly parabolas. Kevin Dandelin questions the rationale behind defining a parabola using the directrix and focus. A response suggests consulting the "Dandelin spheres" section in the Physics Forums Library, which provides a geometric explanation applicable to all conics. This highlights the unifying concept of distance ratios in defining conic sections.

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  • Understanding of conic sections, specifically parabolas
  • Familiarity with geometric concepts related to focus and directrix
  • Basic knowledge of Dandelin spheres and their role in conics
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Students of mathematics, educators teaching conic sections, and anyone interested in the geometric foundations of parabolas and their applications in various fields.

homology
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I suddenly realized that the very notion of a directrix seems odd to me.

(1) I don't know any physical significance of such a thing

(2) How anyone came to define the parabola as such

Any help? I read some older posts about the history of the directrix/focus definitions of the conic sections and I can see the idea of unifying the conics through the ratio of distances from a point to the focus and from the point to the directrix but perhaps there is something more, something deeper, something physical?

Cheers,

Kevin
 
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