spheres & shells, circles & annular rings

Circle, Sphere
The image in the left panel can be thought of as a - - BLUE circle surrounded by a GREEN annulus, or the equator of a - - BLUE sphere surrounded by a GREEN spherical shell. You can vary the thickness of the [annulus,shell] by dragging the yellow dot. How do the curves in the right panel vary with the ratio of the radii? What questions could/would you put to your students base on this applet?