What does UV 400 protection mean on sunglasses?

UV 400 protection on sunglasses means that the lenses block 100% of UVA and UVB rays up to 400 nanometers in wavelength. This level of protection significantly reduces the risk of eye damage caused by ultraviolet radiation, which can lead to issues like cataracts and other eye disorders. Essentially, sunglasses labeled with UV 400 provide optimal protection against harmful sun exposure.