The second benefit of the Polearm Master feat (PHB, p. 168) says:
While you are wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from you when they enter your reach.
The regular rules on opportunity attacks state:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
Obviously, Polearm Master lets you use your reaction to make an opportunity attack when a creature enters your reach, in addition to the opportunity attack any character can make when a creature leaves their reach. However, does the opportunity attack from Polearm Master follow the same restriction on needing to see the creature, or can this attack be made even without seeing the creature that enters its reach?
If an invisible creature enters the reach of a character with the Polearm Master feat, can the character make an opportunity attack against that creature?