Almost every pair of sunglasses sold online in India carries the letters UV400 somewhere in the listing. It is worth knowing exactly what those five characters mean, because the answer is narrower than most product pages imply.
What the number refers to
Ultraviolet light is measured in nanometres. UVB sits roughly between 280 and 315 nm, UVA between 315 and 400 nm. A lens described as UV400 is one specified to block wavelengths up to 400 nanometres — which is to say, the whole ultraviolet band that reaches the ground. That is the entire claim. It says nothing about how dark the lens is, how well it handles glare, or how accurately it renders colour.
Dark and protective are different properties
This is the part that surprises people. Tint is cosmetic; UV filtering is a coating or a property of the lens material. A very dark lens with no UV filtering is arguably worse than no sunglasses at all, because the darkness makes your pupils open wider and lets more unfiltered light through. A lightly tinted lens with proper UV filtering does the protective job perfectly well.
What polarisation does and does not add
Polarisation is a third, separate property. A polarised lens cuts light that has been reflected off a flat surface — a wet road, a windscreen, water. It is a comfort and clarity feature for driving and for being near water. It is not a UV feature, and a polarised lens is not automatically a UV400 lens.
What we can honestly tell you
Where a product page on this site says UV400, that is the specification the supplier states for that lens. We repeat it. We do not run a spectrophotometer at the back of the shop, and we are not going to pretend otherwise — if you need certified measurement, that is a job for an optical laboratory, not for an online shop.
A practical way to choose
Pick the tint for where you will be. A dark grey or brown lens suits open daylight. A gradient lens, darker at the top, suits driving because it shades the sky without blacking out the dashboard. A mirrored lens is mostly a style decision with a small glare benefit. In every case, check that UV400 appears in the specification, and treat everything else in the listing as description rather than promise.