Solution
= Solution
Set $|\Psi\rangle=U|0^m\rangle$. <Unitary conjugation> gives
$$
UR_0U^\dagger
=U(I-2|0^m\rangle\langle0^m|)U^\dagger
=I-2|\Psi\rangle\langle\Psi|.
$$
The operator negates $|\Psi\rangle$ and fixes every vector in $\mathcal S$ <orthogonal> to it. It is therefore the <reflection in a hyperplane> whose normal is $|\Psi\rangle$.