Multivariable: Assume there is an opaque ball of radius 1 centered at the origin.
Suppose that you stand at the point (2,3,0) and look in the direction of a point that is not visible because it is behind the ball. You will then be looking at a point on the sphere.

Find the point on the sphere at which you look if you are looking in the direction of (−2,−3,2).