1. When a part of your eye no longer works so all you see is black, black, and more black.
2. When your eye-sight is really, really, really low.
It is O.K. to make fun of blind people on this site because they cannot read what we're writing, however I would advise you strongly not to do that.
1. I'd rather be deaf than blind.
2. My eye-sight was 200/20 so I was blind.