My eyes are quite sensitive to high altitude sun, especially when I am on snow. I have suffered snow-blindness on several occasions. For this reason, I almost always carry backup sunglasses when I anticipate sustained hikes on snow.
For several years now, I have been using the throwaway sun glasses that ophthalmologists provide after an eye dilation exam as backups. They are just a thin sheet of plastic colored to block UV and bright sun and are designed to fit behind your normal glasses. They can be converted to stand-alone sunglasses with a little duct tape and a rubber band or two. I have obtained a set just by asking at my ophthalmologists reception desk; some offices may charge you for them though.
The ones that I have weigh about a gram and provide me with a bit of security and backup during my clumsy senior moments. I have, once, in the past dropped my sunglasses and watched them slide down a steep snow slope never to be seen again. Now I have an adequate backup.
May I walk in beauty.