Number of conjugacy classes of the reflection in $D_n$. 
Consider the conjugation action of $D_n$ on $D_n$. Prove that the
  number of conjugacy classes of the reflections are 
$\begin{cases} 1 &\text{ if } n=\text{odd} \\ 2 &\text{ if }
 n=\text{even}  \end{cases} $

I tried this:
Let $σ$ be a reflection. And $ρ$ be the standard rotation of $D_n$.
$$ρ^l⋅σρ^k⋅ρ^{-l}=σρ^{k-2l}$$
$$σρ^l⋅σr^k⋅ρ^{-l}σ=σρ^{-k+2l}$$
If $n$ is even, it depends on $k$ if $-k+2l$ will stay even. 
But if $n$ is odd, then at some point $-k+2l=|D_n|$ and therefore you will also get the even elements. So independent of $k$ you will get all the elements. Is this the idea ?

Hint. The Orbit-Stabilizer theorem gives you that $[G:C_G(g)]$ is the size of the conjugacy class containing $g$.  When $n$ is odd, a reflection $g$ commutes only with itself (why?), so $g$ has $[G:C_G(g)]=|G|/2$ elements, which are easily identified as the other reflections.  Now, use this same technique to figure out the answer for the case of even $n$, keeping in mind that dihedral groups $D_{2n}$ have nontrivial centers when $n$ is even (why?).
