Sunglasses Principle
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Sunglasses
The Effects of Sunglasses
Sunglasses block unpleasant glare and protect the eyes from harmful UV rays. This is achieved thanks to metal powder filters that selectively filter incoming light. Tinted glasses selectively absorb certain wavelengths of sunlight thanks to their use of finely divided metal powders (iron, copper, nickel, etc.). In reality, when light strikes the lens, it is attenuated by a process called destructive interference. This means that when certain wavelengths of light (in this case, UVA, UVB, and sometimes infrared) pass through the lens, they cancel each other out on the inside, toward the eye. This overlapping of light waves isn't accidental: the crests of one wave merge with the troughs of its neighbor, resulting in cancellation. Destructive interference depends on the refractive index of the lens (the degree to which light is deflected when it passes from air to different substances) and its thickness. Generally speaking, lens thickness doesn't vary much, while the refractive index varies depending on its chemical composition. Furthermore, sunglasses should not be worn directly in the sun.
Polarized Glasses
The Effects of Wearing Polarized Glasses
The Difference Between Polarized and Non-Polarized Sunglasses
Polarized glasses provide another mechanism for eye protection. Light reflected from asphalt roads is a unique type of polarized light. This reflected light differs from direct sunlight or any artificial light source in its order. Polarized light is composed of waves that all vibrate in a single direction, while ordinary light is composed of waves that vibrate in an unordered manner. This contrasts sharply with the disorderly movement of a group of people versus the orderly marching of soldiers. Generally speaking, reflected light is a well-ordered light. Polarized lenses are particularly effective at blocking this type of light because of their filtering properties. These lenses only allow polarized waves vibrating in a specific direction to pass through, essentially "combing" the light. For road glare, using polarized glasses reduces light transmission by preventing light waves vibrating parallel to the road surface. In fact, the long molecules in the filter layer are oriented horizontally, absorbing horizontally polarized light. This eliminates most of the reflected light without reducing the overall ambient illumination, allowing polarized glasses to be used in direct sunlight.
Photochromic glasses
How do photochromic lenses change color? Why do they change color in sunlight? The lenses of photochromic glasses darken in response to sunlight. When the light fades, they brighten again. This is due to the action of silver halide crystals. Under normal circumstances, they maintain the transparency of the lens. However, when exposed to sunlight, the silver in the crystals separates, and the free silver forms small aggregates within the lens. These small silver aggregates form irregular, jagged blocks that absorb light instead of transmitting it, causing the lens to darken. When the light level drops, the crystals reform, and the lens returns to its bright state.
