Sunglasses can shield your eyes from uncomfortably bright light while also protecting your eyes from the harmful effects of ultraviolet rays.All this is thanks to metal powder filters that “select” light as it comes in.The tinted glasses selectively absorb some of the wavelengths that make up the sun’s rays because they use very fine metallic powders (iron, copper, nickel, etc.).In fact, when light hits the lens, it is subtracted by a process called destructive interference.That is, when certain wavelengths of light (UVA, UVB, and sometimes infrared) pass through the lens, they cancel each other out on the inside of the lens, toward the eye.The overlapping of light waves is no accident: the crest of one wave and the trough of a nearby wave combine to cancel each other out.The phenomenon of destructive interference depends on the refractive index of the lens, which is how much light diverges as it passes through different substances from the air, and also on the thickness of the lens.Generally speaking, the thickness of the lens does not vary much, while the refractive coefficient of the lens varies according to the chemical composition.And sunglasses should not come into direct contact with the sun.