The worst detentions are when he goes to Snape about his glasses. He's broken them three times this year: dropping them off a staircase, falling off his broom, and once leaving them out on the table where Crabbe always dropped his books.
The first time, he scrubbed cauldrons for an hour before Snape said, "oculis reparo," and gave his glasses back. Even when Snape wasn't watching - which was not often enough - he felt much too open and obvious. He tried not to look up at all.
If Snape knew that the glasses meant anything more than homework and not tripping over his own feet, he wouldn't fix them right. Harry needs his glasses, and not just to see. They're a shield: they keep Harry on one side and everyone else - the ones who mean well and the ones who don't - out there. All the lines are blurrier without them.
The second time, Snape didn't fix them. Harry was convinced his broom had been tampered with, so he did it himself. The cracks disappeared with a snapping sound, and Harry felt brilliant. Until he put them on and realized he'd repaired them right into flat panes of glass.
"Oh, hell," he'd said, and Draco started laughing at him. "No wonder you haven't tried fixing your eyes, Potter, they'd pop right out of your head!" It wasn't very funny, but Harry joined in because it was a relief to be on the ground, and his glasses the only thing broken.
He took them to Snape again, who gave him a concave hand mirror to practice on in his detention, but had to send his glasses off anyway. Snape didn't know his prescription, and couldn't make the lenses curve again. Everyone seemed to be watching him, that week.
The third time, Harry got another hour's practice breaking the mirror that never repaired itself in the same way twice and kept reflecting things that weren't in front of it. Once, there were eyes that glowed. Harry dropped the mirror and cast again.
Long after he's started mending his own glasses, he will learn about mercury. "Used in all wizarding and many Muggle mirrors," MacGonagall lectures, "Quicksilver interferes with transfiguration spells, particularly reparo. Broken mirrors should be discarded immediately."