In this problem, we use the Method of Images and get the resulting properties of the charge distribution on the plane. At the end of this process, Griffiths, in Introduction to Electrodynamics, comments that we could have said that the charge on the plane was going to be $-q$ with some hindsight. How can this result have been obvious before arriving at the solution mathematically? I can't think of any thought process that would lead to the same conclusion.
Best Answer
I think that the "hindsight" goes as follows?
Electric field lines start on a positive charge and end on a negative charge so where else can the electric field lines from the $q$ charge go but to the infinite grounded conducting plate?