light dawns (on one)
To understand or realize something with sudden clarity or certainty. The light finally dawned when I remembered that my grandfather had been stationed in Japan many years ago. She'd been cheating on me for months, but it was only when he left his tie in the house that the light dawned on me.
If light dawns, you suddenly realize or understand something. I didn't realize they were a couple till I saw them together last night and the light suddenly dawned. Note: You can also say that light dawns on someone. `Oh!' she said, as if the light had finally dawned on her. `I'm on the wrong floor, huh?'