Zefal Z-Eye Helmet Mirror (Black)

This is the best mirror out there; I have tried many.
It is _convex_, this increases the field of view significantly.
Convex surface plus helmet mount means you can see the whole area behind.
Slide it up so it is just forward of the visor, then clip off the extra length.
The oblong shape is near-optimal.
It has a fine-angle ball-adjuster plus a gross-angle adjuster that clicks.
The mirror can be wound up and out of the way if you need to put your helmet say, in a backpack.
Buy a small phillips (+) screwdriver (the electronics type) to adjust the tension of the little ball.
A couple zip-ties help reinforce the mount on the helmet.
Flat mirrors are like looking through a soda straw; get this thing.
Cycling without knowing what is coming 50 meters back is nuts.

I use it couple years and. Only problem it plastic so it got scratched up with age and harder to see now. otherwise its good.