Physical meaning of directrix?

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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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tiny-tim
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