Opinion is divided on their utility, but mirrors are found in a number of forms:

• handlebar mounted

• D+D Oberlauda UltraLight Bike Mirror *

• Mirrycle bar-end mirror *

• Ortlieb Ultralight Mirror * **

• BicycleStore® Adjustable Universal Handlebar Rearview Mirror (Amazon) *

• head mounts (helmet or glasses) *

• Third Eye On-Lens Micro Bicycle Mirror *

• Bike Peddler Take A Look Cycling Mirror *

• other

http://www.icebike.org/what-you-need-to-know-about-cycling-mirrors/