Sunglasses can block the uncomfortable light, while protecting the eyes from UV damage. All of this is due to the metal powder filter, which can be "selected" when the light enters. Colored glasses can selectively absorb part of the composition of the sun's rays, because it uses a very fine metal powder (iron, copper, nickel, etc.). In fact, when the light shines on the lens, the light is subtracted based on the so-called "destructive interference" process. In other words, when some wavelengths of light (here refers to the ultraviolet a, ultraviolet b, and sometimes infrared) through the lens, the inside of the lens that is toward the direction of the eyes, they will cancel each other. The formation of overlapping waves of light is not an accidental phenomenon: the wave of a wave with its close to the wave of the trough together, resulting in offset each other. The destructive phenomenon depends on the refractive index of the lens (ie, the degree of deviation of light from the air through the different substances), but also on the thickness of the lens. In general, the thickness of the lens changes little, while the lens refraction coefficient is based on differences in chemical composition and different.