Sunglasses, also called sun-shading mirrors, are used for sun-shading. People usually adjust the luminous flux by adjusting the pupil size in the sun. When the light intensity exceeds the adjustment ability of the human eyes, it will cause harm to the human eyes.
Therefore, in outdoor activities, especially in summer, it is necessary to use sunshade mirrors to block the sun to reduce the fatigue caused by eye adjustment or the damage caused by strong light stimulation.
Sunglasses can block uncomfortable glare, and at the same time can protect the eyes from ultraviolet rays. All this is due to the metal powder filter, which can "select" the light when it hits it. Colored glasses can selectively absorb some of the wavelengths that make up the sun's rays because they use very fine metal powders (iron, copper, nickel, etc.). In fact, when light hits the lens, the light is reduced based on the so-called "destructive interference" process. In other words, when certain wavelengths of light (herein referred to as ultraviolet a, ultraviolet b, and sometimes infrared) pass through the lens, they will cancel each other out on the inside of the lens, that is, toward the eye. The overlapping of light waves is not accidental: the peaks of one wave and the troughs of the adjacent waves combine to cancel each other. The phenomenon of destructive interference depends on the refractive index of the lens (that is, the degree of deviation when light passes through different substances in the air), and it also depends on the thickness of the lens. Generally speaking, the thickness of the lens does not change much, and the refractive index of the lens varies according to the difference in chemical composition. And the sunglasses should not be in direct contact with the sun.

Effect
Ultraviolet rays can damage the cornea and retina, and high-quality sunglasses can completely eliminate ultraviolet rays.
When the eye receives too much light, it will naturally shrink the iris. Once the iris shrinks to its limit, people then need to squint. If there is still too much light, such as sunlight reflected from snow, it will damage the retina. High-quality sunglasses can filter out up to 97% of the light that enters the eye to avoid damage.
Certain surfaces, such as water, can reflect a lot of light, and the bright spots produced in this way can disturb the line of sight or hide objects. High-quality sunglasses can use polarization technology to completely eliminate such glare. We will introduce polarization technology later.
Certain frequencies of light will blur the line of sight, while other frequencies of light can enhance contrast. Choose the right color for the sunglasses, so that it can get better results in a specific environment.
If sunglasses do not provide UV protection, they will expose you to more UV rays. Cheap sunglasses filter out some of the light, causing your iris to open up to receive more light. This will also allow more ultraviolet rays to enter, increasing the damage caused by ultraviolet rays to the retina.
Therefore, there is indeed a difference between various sunglasses. For a specific use environment, choosing suitable and high-quality sunglasses will provide you with the greatest protection.