I wear glasses (and wearing eye protection when riding is always a good idea anyway) and I use an eyeglass-mount mirror (the "Take-a-look'). I wouldn't ride without it.

It requires some practice to master the partial head-turn that lines up the mkirror with the road behind you, but this quickly becomes second nature.

It's not a substitute for turning your head. But it's invaluable for merging leftwards (or rightwards if you're UKish), and for doing a quick check-six whenever approaching a place where a car coming up behind you might be about to pass you and turn right.

I tried a helmet-mounted mirror and didn't like it nearly as well. The field of view was smaller, the mirror jiggled more, and it seemed harder to line up.

RichC