Take A Look Cycling Mirror – Low-Tech Rearview Mirror for Bikes – Fits Helmet & Sunglasses – Shatter-Resistant Polycarbonate Lens, Anti-Vibration Stainless-Steel & Brass, Always On (Left-Side)

I love this mirror that clips onto any glasses, I won't ride without it. Great for riding in traffic and changing lanes, and also great for riding with friends to make sure you're not leaving them behind. https://www.amazon.com/Take-Look-Cycling-Mirror-Shatter-Resistant/dp/B000C17M26