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The ellipse has two
particular points, called focuses, which are on the longer diameter.
The focuses have the
following property:
The sum of the
distances of any point of the curve from
the focuses is constant.
Another property of the focuses is that
the perpendicular to the ellipse
at any of its points halves the angle formed by the segments
joining this point to the two focuses.
Consequently, a ray of light, starting from one of the two focuses and reflected on the ellipse,
passes through the other focus. The same goes for
sound waves.
If somebody speaks, or even whispers, standing in one of the focuses
of an elliptic vault room, the sound waves will be reflected on the vault
and will gather together again in the other focus,
where they can be heard by whoever is standing there.
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