The principle of sunglasses.

Sunglasses can adjust the external light to make it suitable for human eyes.

A beam of light has a certain amount of electromagnetic energy. The size of the wave is measured by wavelength. The wavelength range of visible light is 400 to 700 nanometers.

The energy of light waves is inversely proportional to the wavelength, the shorter the wavelength, the higher the energy. Among visible light, violet light has the largest energy, red light has the smallest energy, and above the visible light spectrum is ultraviolet light. In fact, natural light contains a lot of ultraviolet rays.

Due to the high energy of ultraviolet rays, ultraviolet rays can cause damage to your cornea and retina. The unit of measurement for the brightness or intensity of light is lumens.

For example, indoors, most artificial light has an intensity of 400 to 600 lumens. If you are outdoors in a sunny day, the brightness will fluctuate in a large range. The brightness in the shade is 1000 lumens, and in places with large concrete floors such as highways, the brightness is 6000 lumens.

Below the brightness of 3500 lumens, our eyes feel very comfortable. When the brightness of direct or reflected light reaches 4000 lumens, the human eyes begin to struggle to receive light. When we try to look at these very bright areas, we only see white flashes, which is glare. In order to reduce the discomfort caused by a large amount of light entering the eyes, we began to squint.

Once the brightness reaches 10,000 lumens, your eyes begin to filter out the light completely because of the intensity of the light received. Exposing to such a strong light for too long can cause damage and cause temporary or permanent blindness.

On a clear day, the brightness of light reflected by a large area of snow can be as high as 12,000 lumens. If we look directly at the light without any protection measures, it will lead to "snow blindness".

Visible light is the light that the human eye can perceive.

The visible light we see from the sun looks colorless, which is what we often call white light. In fact, it is a mixture of colored lights of multiple frequencies.

The mixed light of all colored lights in the visible light spectrum is colorless, that is, white.