the ultraviolet absorption of the atmosphere

Buy a good pair of sunglasses is more difficult than you think. This is not enough, you see them. Apart from the aesthetic point of view, there are several things to consider. These include the amount of ultraviolet (UV) protection for their material, and how you will use them.
UV protection is very important, because the sun's rays can seriously damage your eyes. Although most of the ultraviolet absorption of the atmosphere, UV-B and UV-A have no. , UV-B ray is fatal, although both UV-A and UV-B can lead to cataract and age-related macular degeneration and other eye problems host - both of which can lead to blindness.
For your safety, wearing dark glasses, the block of 99% to 100% UV-A and UV-B ray. Contrary to popular belief, dark hurt lens will not protect your eyes from ultraviolet, unless they say. The color of the lens has no effect of ultraviolet ray, don't be misled.
Some contact lenses may prevent ultraviolet ray, but their protection level is not as good as sunglasses. Labeled "UV 400, block all light at a wavelength of 400 nm model. Wraparound Sunglasses provides an additional layer of security, because they prevent the harmful ultraviolet frame into the surrounding. They can also protect the eyes from dirt and wind.
Whether you use a glass or plastic lens, depends entirely on what you do. Glass lenses, optical clarity is the best and the scratch is not easy, but they than plastic, and easily broken. Plastic lens lighter, easier to break, but they are easy to scratch.
If you do a lot of outdoor sports, can consider to buy a pair of sports goggles. Although all sunglasses is by the United States Food and Drug Administration requirements impact resistance, sports goggles they not so strong is made of polycarbonate - using the same material you want material in aircraft windshield glass. This is 50 times stronger than the glass lenses.