why they are purely cosmetic

Scratch resistant scratch resistant lenses, although not completely (such as glasses lens material not scratch test) do the lens should be more resistant to. Glasses lens scratch resistant treatment, with a clear, hard coated lens front and back. Mirror coatings is clear, can be in a variety of colors and different anti reflective coating, including copper, gold, silver, blue, pink and green heat to name just a few. Mirror glasses (state troopers often see) does not change the way, you see, why they are purely cosmetic. Finally, UV (ultraviolet) glasses lenses block ultraviolet paint harmful, is believed to be caused by cataract, retinal damage and other eye problems. Glasses lens does not change the appearance of the UV curable coatings.
The coating can be applied to glasses lens options, can help reduce glare and eye protection against solar ultraviolet radiation (UV) harmful. Polarized lenses have been reduced to one of the most popular choice of glare water boat and fishermen for years.
Now, polarizing glasses you can find all kinds of glasses wearers were used including drivers, skiing, golf, cycling and jogging, because they eliminate glare some outdoor activities. For driving, they help to reduce glare, a flat and long road, give you a better vision. By blocking is polarized glasses lens polarizer horizontal polarization and vertical orientation of the light work.
From shallow to deep photochromic lens depends on the amount of UV (ultraviolet) exposed to light, they are changing. The lens to be used is made of glass, but now with conventional plastic, polycarbonate (impact) and high refractive index glass and plastic. When exposed to UV light, the lens of silver halide, make them dark. In all kinds of glasses lens ever thinner, lighter, some photochromic glasses from clear to dark for 60 seconds. The design can be found, in almost every shot, refractive index and prescription one hundred percent UV lens.