This Geogebra applet is about exploring the intersection curve of a torus with a plane.

The file can be seen in Geogebra’s material repository (https://ggbm.at/MmTVuXYk) or, better, downloaded as a “.ggb” file (https://www.geogebra.org/material/download/format/file/id/Vp95FtPQ) and run locally on the PC. The free *Geogebra Classic* desktop program is available at this page.

**The plane
**

The plane can be positioned using the parameters , and that set the position of the normal vector through the origin.

is the direction in the horizontal plane (azimuthal angle), is the direction with respect of the horizontal plane (elevation angle) and is the modulus (length) of the vector.

The components of the vector are then .These are also the coordinates of the point , projection of the origin on the plane.

The Cartesian equation of the plane is then:

.

and its alternative parametric equations are:

With above parametric equations the parameters and can be interpreted as the *embedded* orthogonal Cartesian axes in the plane starting from the point (plane origin), where is the horizontal axis (parallel to the plane) and is the vertical axis in this plane (perpendicular to ).

**The torus**

The torus position is fixed, with center in the origin and the axis as axis of symmetry (or axis of revolution). Anyhow its parameters (major radius) and (minor radius) can be changed through the respective sliders.The parametric equation of the torus surface is:

Alternatively, the torus Cartesian equation is:

**The views**

In the *3D graphics* (bottom frame) there is the spatial representation of the plane, the torus and the intersection curve. Click and drag the mouse to change the point of view.

In the *Graphics* view (upper left frame, the one with the sliders) is represented the intersection curve as seen in the intersecting plane. Note that it won’t change with changes of the angle as this parameter just rotates the plane around the torus symmetry axis.

In the *Graphics 2* view (upper right frame) there is a representation of the 3D view projected in the plane. The slanted lines showed here are a visual aid to help imagine how the plane develops in the invisible dimension. They are:

- intersection of the plane with the plane ;
- intersection of the plane with the planes (the horizontal planes tangent to the torus above and below);
- intersection of the plane with the plane that is, the projection of the axis on the plane.

**References**

https://en.wikipedia.org/wiki/Torus

https://en.wikipedia.org/wiki/Toric_section

*Last edited: 24 july 2017*