Let R and r be the external and internal radii of a hallow cylinder and h be its height as shown in figure. Then

Now, consider a rubber ball made of thin rubber sheet. Clearly it is neither a hallow sphere nor a solid sphere. However, it can be considered as the difference of two solid concentric spheres. We call it as spherical shell as defined below.

Spherical shell: The difference of two solid concentric spheres is called a spherical shell.

A spherical shell has a finite thickness, which is the difference of the radii of the two solid spheres which determine it.

If 'R' and 'r' are respectively the outer and inner radii of a spherical shell, then
Outer surface area = 4πR²
Volume of material = π(R³ – r³)