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# What is a Paraboloid?

A paraboloid is a captivating 3D surface, shaped like a parabola extended in another dimension. Imagine a satellite dish or a liquid swirling in a glass – both embody the paraboloid's unique geometry. This form is not just aesthetically pleasing but also structurally efficient, optimizing material use. Curious about its applications in everyday life and technology? Let's explore further.
James Doehring
James Doehring

A paraboloid is a particular kind of three-dimensional surface. In the simplest case, it is the revolution of a parabola along its axis of symmetry. This kind of surface will open upwards in both sideways dimensions. A hyperbolic paraboloid will open upward in one dimension and downward in the other, resembling a saddle. Like in a two-dimensional parabola, scaling factors can be applied to the curvature of a paraboloid.

To understand how a paraboloid behaves, it is important to understand parabolas. Indeed, some cross sections of a paraboloid will form a parabola. The equation y = x2 will form a parabola in a standard coordinate system. What this equation means is that the distances of a point on this line from the x- and y-axes are always going to have a special relation to each other. The y value will always be the x value squared.

If one revolves this line around the y-axis, a simple circular paraboloid is formed. All vertical cross-sections of this surface will open up in the positive y direction. It is possible, however, to form a hyperbolic paraboloid that also opens downward in the third dimension. Vertical cross sections in this case will have one half of their parabolas opening in the positive direction; the other half will open in the negative direction. This surface of a hyperbolic paraboloid will resemble a saddle and is called a saddle point in mathematics.

One application of the paraboloid surface is the primary mirror of a reflecting telescope. This kind of telescope reflects incident light rays, which are nearly parallel if they come from very far away, to a smaller eyepiece. The primary mirror reflects a large amount of light to a smaller area. If a circular mirror is used, reflected light rays will not perfectly match up at a focal point; this is called spherical aberration. Though more complicated to make, parabolic mirrors have the geometry required to reflect all light rays to a common focal point.

For the same reason as in the parabolic mirror, satellite dishes commonly use a concave parabolic surface. Microwave signals sent from orbiting satellites are reflected off the surface toward the dish’s focal point. A mounted device called a feedhorn then collects these signals for use. Sending signals operates in a similar way. Any signal sent from the focal point of a paraboloid surface will be reflected outward in parallel rays.