Actually, wearing more than one pair of glasses is actually quite useful. When you get asked a hard question by a student you can do something like:
Say "Well.....". Take off glasses thoughtfully, clean them, put them back in the case, take out other pair of glasses, clean them even more thoughtfully and carefully, put them on, stare intently at the computer screen for a few seconds and press a few keys. Tthen take off close up glases, put on normal glasses, turn to face student and continue "....that all depends."