Why is inversion in a straight line the same as reflection? In the inversive plane points $P$ and $P'$ are defined to be inverses with respect to a circle $\omega$ of radius $k$ and center $O$ when they are distinct from $O$, on the same ray emanating from $O$, and $OP * OP' = k^2$. When $\omega$ is a straight line (or circle of infinite radius), the operation of inversion is the same as a reflection (across the line). However, since the equation $OP * OP' = k^2$ breaks down with $O$ becoming the point at infinity when $\omega$ is a straight line, I am having trouble seeing the precise argument that is made to justify the equivalence of inversion and reflection in this case. Can anyone supply this argument?

After researching this a bit I found a couple of explanations, one of which makes the most sense to me. However, one thing that is clear as Thomas Andrews points out in his answer, is that it is ultimately a definition, not a theorem. However in some ways it makes most sense to have this definition be the definition of inversion itself. If inversion is (equivalently) defined as below, then the equality of inversion and reflection does become a theorem (I believe).
Explanation 1: It is possible to define inversion completely in terms of orthogonality: "Any point on $\omega$ is its own inverse, the inverse of any other point P is the second intersection of any two circles through P orthogonal to $\omega$. Replacing $\omega$ by a line, we deduce that reflection in a line may properly be regarded as a special case of inversion in a circle." (from Coxeter and Greitzer, Geometry Revisited). The easiest way for me to understand this explanation is to use this diagram.

Another explanation (from Pedoe, "A Course of Geometry") uses the concept of Harmonic Conjugates. The basic idea is if $A$ and $A'$ are diametrically opposite points on $\omega$ (with $\omega$ a circle, not a line), any pair of inverse points $(P, P')$ are harmonic conjugates with respect to $A$ and $A'$. As the radius of $\omega$ goes to $\infty$, $A'$ moves to $\infty$ along with O and $(P, P')$ become image points. I must admit that I don't understand this explanation for the same reason that I had trouble with my original conundrum: You wind up with saying something like $\frac{\infty}{\infty} = 1$ (or equally absurd).
