If you use a mirror to reflect half of the light back on itself, then you get all the momentum of the original beam -- the mirror (and whatever it's attached to) acquires 2 * (1/2 * p) = p momentum for reflecting half of the beam backwards.
(you; use; a mirror; to reflect half of the light back on itself) (it; 's attached; )
you it
use 's attached
a mirror to reflect half of the light back on itself