"And Pharaoh said, Who is the LORD...?"

Exodus 5:2 records Pharaoh's response to Moses's and Aaron's informing him of G-d's instruction: "And Pharaoh said, Who is the LORD...?"

My friend Gershon G once answered the same question when I put it to him - in response to "who is G-d?", he responded, "who are you?".

Perhaps this gives us a clue as to why G-d brings about the 10 plagues. Could they tell us more about who Pharaoh was than about who G-d was?