What is a 3D half sphere called

A 3D half sphere is called a hemisphere, derived from the Greek words "hemi" (half) and "sphere." A hemisphere is created by cutting a complete sphere along any plane that passes through its center, resulting in two identical halves, each with a flat circular base and a curved dome-like surface. The equator on Earth, for example, divides the planet into the Northern and Southern Hemispheres.

Hemispheres have unique geometric properties with specific formulas distinct from full spheres. The curved surface area of a hemisphere is 2πr² (half the sphere's 4πr²), but the total surface area including the flat circular base is 3πr² (adding πr² for the base). The volume of a hemisphere is (2/3)πr³, exactly half the full sphere's volume of (4/3)πr³. Hemispheres appear frequently in architecture (domes on buildings like the U.S. Capitol or Pantheon), engineering (pressure vessel caps, satellite dishes), and nature (bubbles resting on surfaces). When calculating materials or properties for hemispheres, remember to include the flat base in surface area calculations if you're covering the entire shape, but use only the curved portion if you're specifically dealing with the dome section. The hemisphere's combination of a curved surface meeting a flat base creates unique structural properties—domes can distribute weight efficiently, making them popular in architecture for creating large open interior spaces without columns.