Desdemona goes to put a handkerchief to her husband. She has heard that his head aches. He pushes it away and the handkerchief falls to the floor.
Neither of them notices, and they leave the room. In chapter 48 what Othello demanded was proof the eye could see. That proof is lying on the floor now.
Emilia comes into the room and picks it up. She remembers that her husband has asked for this handkerchief many times. She does not ask what it is for.