1. Two Ways Sunglasses Protect Your Eyes From Glare
Standard colored sunglasses work by weakening all the light that enters your eyes, which reduces brightness [6]. With these lenses, glare and the rest of the scenery become equally darker. In contrast, polarized lenses selectively block light that has reflected off surfaces like water or roads [6]. When comparing lenses of similar darkness, it is explained that polarized lenses are better at reducing road glare. This article first looks at tinted lenses, then examines why reflected light can be selected and blocked.
2. How Tinted Lenses Weaken Light and How Dark They Are
Tinted lenses absorb part of the light to weaken it. The amount of light allowed through is measured as visible light transmittance, expressed as 0–100%; a lower number means a darker lens [7]. The color of the lens also changes how things look. According to retailer explanations, yellow lenses reduce blue light to increase contrast, while gray lenses reduce glare while keeping colors true. These are rough guidelines, not official standards [7].
People often think that darker lenses block more ultraviolet (UV) rays. However, a product test by Kanagawa Prefecture examined 22 types of commercial sunglasses. The visible light transmittance ranged widely from 9% to 96%, but UV transmittance was not related to how dark the lens was; all products met their labeled UV values [8]. This result applies only to the products tested in that survey, not all products. The prefecture recommends trying on sunglasses rather than choosing based on darkness alone.
3. Why Reflected Light Vibrates in One Direction
Light travels as a wave, and the direction in which the wave vibrates is called "polarization." Sunlight contains waves vibrating in all possible directions. However, when light reflects off a smooth surface like water, the reflected light becomes polarized in a direction parallel to the surface (horizontal for water) [5]. At a specific angle of incidence, known as Brewster’s angle, the reflected light is completely polarized [5].
In 1938, Edwin Land noted that most glare on sunny days comes from sunlight reflecting off horizontal surfaces like sidewalks and paved roads [10]. If you block only the horizontally vibrating light, you can reduce this reflection.
4. How Polarized Lenses Block Glare Like a Fence With Narrow Gaps
Polarizing films are often explained using the analogy of a "fence with narrow gaps." Light vibrating at a right angle to the gaps cannot pass through, while light vibrating along the gaps can [2]. This is just an analogy; there are no actual gaps. The basic structure involves stretching long chain-like molecules in a material to align their direction, which absorbs vibrations in that aligned direction [3].
A common modern type uses polyvinyl alcohol (PVA) polymer containing iodine, which is stretched to align the molecules [4]. Light vibrating parallel to the molecular chains is absorbed, while light vibrating perpendicular to them passes through [4]. Since reflected light from water is horizontally polarized, a lens configured to pass only vertical light will block much of that reflection [1]. In reality, it is difficult to align all long molecules in exactly the same direction [3].
5. How Edwin Land Made Polarizing Film Practical in 1929
Crystals of a material called herapathite, which selects light direction, were discovered in 1852. However, large crystals could not be made, so practical use was difficult for about 75 years [10]. In 1929, American Edwin Land is introduced as having patented the world’s first practical polarizing sheet that could be mass-produced cheaply. His method involved aligning tiny herapathite crystals with a magnetic field and placing them on plastic [10]. This magnetic method was used initially in 1929 but was later replaced by the stretching method.
In the 1930s, Land negotiated with the American Optical Company to manufacture polarized sunglasses, and the Polaroid Corporation was established in 1937 [11]. Because sources differ by about one year on the exact invention date, this article refers to the "1930s."
6. Where Polarized Lenses Work Well and Where They Work Poorly
Polarized lenses (and camera polarizing filters) reduce reflections from water, glass, and leaf surfaces. They may be less effective on metal or mirror reflections [2]. The effect is highest at an angle of about 30–40 degrees relative to the surface [2]. For water, Brewster’s angle is about 53 degrees, so on a calm freshwater lake, the lenses work best when the sun is about 37 degrees above the horizon [3].
There are drawbacks. Devices like LCD screens emit polarized light themselves, so polarized lenses can interfere with the screen’s orientation, making it hard to see. Stacking two polarized lenses can also make things hard to see [1]. In Japan, under the Household Goods Quality Labeling Act, standards state that a product can be labeled "polarized sunglasses" if its polarization degree is 90% or higher and the axis deviation is 15 degrees or less [9]. For safety, JIS standards state that lenses with visible light transmittance of 8% or less should avoid use for driving or on the road, and lenses with less than 75% transmittance should avoid use for driving or on the road at dusk or night [7].
Try It Yourself
If a family member has polarized sunglasses, borrow them and look at a smartphone or computer screen while slowly rotating the lens. Depending on the screen type, you may find an angle where the screen becomes dark. Think about what causes this difference. On a sunny day, look at reflections in puddles or window glass through polarized lenses and rotate them. Observe if there is an angle where the glare changes intensity. In any case, never look directly at the sun. Official information on labeling standards and test results can be found on the Consumer Affairs Agency’s "Sunglasses" page and the Kanagawa Prefecture product test report.