A mineral sunscreen, composed of an assembly of mineral microparticles or nanoparticles, most often zinc oxide or titanium dioxide, diffracts light and reflects UV rays, like a mirror. 

UV A and B correspond to non-visible light. UVA corresponds to a wavelength of 400 to 315 nm, UVB to a wavelength of 315 to 280 nm. UVA (much more numerous) penetrates deeper into the dermis, and is mainly responsible for pigmentation spots, skin aging and wrinkles. UVB rays (only 2% of all rays) penetrate less deeply (to the epidermis) and are responsible for sunburn, brown spots, burns, blisters and most skin cancers. Glass blocks UVB, but not UVA. This means you can still tan (very lightly) behind glass, so remember to apply a sunscreen or moisturizer that protects against UVA every day, even if it's cloudy, or you're at home and not exposed to direct sunlight. 

By applying sunscreen, you protect yourself in the short and long term. This means that a tanned complexion also needs sun protection to combat oxidation and photo-aging. It is therefore strongly recommended to apply sunscreen even if you have a tanned complexion.