What is a liquid mirror?
As we all know, a perfect reflecting parabola represents the ideal contour for a mirror to focus parallel rays of light into a single point. It is therefore of the greatest importance to have realized that a rotating liquid surface takes the shape of a parabola under the constant pull of gravity and a centrifugal acceleration, which grows stronger as the distance from the central axis increases (E. Cappoci, 1850). The parabolic shape occurs because a liquid surface always sets it local surface perpendicular to the net acceleration it experiences, which in this case becomes stronger and less inclined with distance from the central axis (see Figure 1).
