Sunglasses with UV lens block out invisible light waves from the sun called ultraviolet radiation, thus the acronym UV. There are three types of UV rays from sunlight: UVC, UVB, and UVA. Too much exposure to UV rays, which are not visible to the naked eye, can create or worsen certain conditions such as pterygium, cataracts, and pinguecula and can even cause structural damage to retina, eye lens, and cornea.
UVC rays are the most intense rays at 100 to 280 nanometers (scientific measurement of light wavelengths), and would be considered the most harmful to your eyes if it weren't for the fact that our ozone layer blocks out just about all UVC rays. The debate on ozone depletion has resonance here, however, since a deteriorating ozone layer means the potential for dangerous UVC rays to pass through.
UVB rays are less intense than UVC rays, at longer wavelengths (280-315 nm), but are considered the most harmful rays by the optical community, being that these are the rays that partially penetrate our ozone layer, and also are the type of radiation that causes sunburn and skin cancer at highest intensities as well as pterygium and pinguecula. Overexposure to UVB rays also contribute to premature aging.
UVA rays are the least intense of the three (at 315-400 nm), but can still get to the eye cornea, lens and retina, potentially contributing to the development of cataracts and macular degeneration.
Sunglasses with UV400 is especially made to block UV light in wavelengths up to 400nm. It is important to have sunglasses with this measure of protection. Polycarbonate lenses are also inherently protective against UV rays, by virtue of its very composition. The majority of Olympic Eyewear bulk sunglasses are made with polycarbonate lens, and they all comply with US and International UV Protection Standards. They can be found in the latest styles from brands like DG Sunglasses, X-Loop Sunglasses and Biohazard Sunglasses.