By protecting the eyes against UV radiation, sunglasses perform a very important function. Everyone appreciates the benefits of a good pair of eyeglasses especially when they are UV coated and polarized.
Do not buy cheap sunglasses that say they provide 100% UV protection, because they probably do not. But cheap sunglasses will provide some level of protection against UV rays, even if it is as low as 40%. When you buy your sunglasses from reputable dealers and if UV 400 is mentioned on the glasses, it actually means that the sunglasses provide 100% protection against UV rays. If you still doubt it, you can take it to the store you bought it from, where professional machinery might be available to test the UV protection, such as a UV meter.
If your sunglass is said to be made up of CR-39 plastic, nearly 88% of the UV radiation that falls on it will be absorbed. If your sunglass is made of polycarbonate material, nearly 100% of the UV rays that fall on it are absorbed.
It you buy your sunglass from a reputed store, and if it is a good quality polarized sunglass, there is a high chance that it is also UV protected. Polycarbonate lenses will usually absorb UV rays. However many sunglasses (cheap ones) use triacetate. About 40% of the UV rays that fall on them are absorbed. The lens may also be made from certain acrylic materials that can absorb UV rays partially or fully.
If you want sunglasses that offer 100% protection from UV rays, you have to buy them from reputed stores. Such stores will also give you a guarantee card that states the lenses are 100% UV protected. Such stores will also keep a photometer that can tell you if the lenses are providing the UV protection or not.