Image caption, ...you've done it! The sectors rearranged into the rectangle show the area of the curved surface of the cone = π × radius × slant length.

(Image: Wikimedia Commons, CC SA 3.0) In materials with Dirac cones, the valence and conduction bands take the shape of the upper and lower halves of a conical surface, meeting at points called Dirac ...

Image caption, ...you've done it! The sectors rearranged into the rectangle show the area of the curved surface of the cone = π × radius × slant length.