Why can this sunglasses be anti -glare?

Author:Institute of Physics of the Ch Time:2022.09.27

This article is produced by Light Popular Popularity

Writing: Wang Xiaojie (School of Physics Science, Nankai University, Physical Science Popularization Base Nankai University)

Review expert: Gao Hui (Huazhong University of Science and Technology)

As an office worker, driving is our common way of travel. During the day, the sun is strong. Many people choose to use a sunboard to cover the sun. Not only is it troublesome to get around, but it will also block the sight. And strong light makes the eyes discomfort, causing dangerous driving.

Therefore, "sunglasses" have become the "essential artifact" of the driver. However, ordinary sunglasses can protect ultraviolet rays and blue light, but they are helpless to glare. The light reflected from the horizontal surface of the road, water, snow, or front glass of the car will cause glare, and it will seriously affect the sight of the driver's eyes, and the short -term vision blind area can easily cause traffic accidents.

As a result, polarized sunglasses have become a better choice for driving people, which can effectively reduce glare. However, there is a scene that needs to be particularly emphasized: in the case of insufficient light, such as at night, especially when driving, wearing any type of sunglasses will weaken the light and affect the field of vision, so it is not recommended to wear it at night.

Before talking about polarized sunglasses is more amazing than ordinary sunglasses? why is that? Today, let us explore the secrets of polarizing sunglasses together.

Figure 1 (cover map): polarized sunglasses

Photo source: Light Popular Popularity Square/Veer

Literally, the difference between polarizing sunglasses and ordinary sunglasses is mainly reflected in the word "polarization". What mysteries are hidden behind this simple word? To figure out the secrets of polarizing sunglasses, we need to first understand the core structure of the polarized sunglasses -polarizing tablets.

1. The core structure of polarized sunglasses -polarizing tablets [1]

Modern optical theory tells us that alone is an electromagnetic wave. Since it is a wave, is it horizontal or vertical waves?

In 1809, French physicist Etienne Louis Malus discovered the polarizing phenomenon of light, proved that light was a horizontal wave, which indicated that the vibration direction of the optical vector and the direction of the transmission of light vertically. The electrical vector in the two -dimensional space vertically in the direction of the communication has a variety of vibration state, which is called the polarized or polarized structure of the light.

According to the changes in the size and direction of the light wave electrical vector, the light can be divided into natural light (Figure 2A), the line partial vibration (Figure 2B), and some polarized light (Figure 2C). The common light sources in our lives belong to natural light, such as sunlight, lighting, etc. The vibration direction of the light is chaotic. The macro seems to contain vibration along all directions. The interface reflection will be converted into line polarizing light or partial polarized light. The direction of line polarizing light is usually very fixed, only contains a single vibration direction, and some polarized light is from the light between natural light and line polarization light. The vibration strength in different directions is different. After understanding the different states of light, we can use this theoretical analysis of the specific process of polarizing sunglasses.

Figure 2: Several common light vibration distribution (A) natural light; (B) line is polarized; (C) partial polarization light. Picture source: Wang Xiaojie

The horizontal wave characteristics of light are similar to the horizontal wave characteristics in mechanical waves. We can use the polarization of light to simulate the light. The reason why polarizing sunglasses can filter glare is because polarizing sunglasses only absorb light in a specific direction, and light can be passed (Figure 3A); while vibration in other directions is absorbed more, light passes through through the light, and the light passes through the passage. Few (Figure 3B). This properties are called binary colors, and the optical element device with binary color is called polarizing tablets. Among them, the vibration direction of the light can be called transparent direction.

Figure 3: The vibration direction of the polarization phenomenon of the mechanical wave analog light (A) wave is parallel to the slit; Picture source: Wang Xiaojie

Under normal circumstances, the polarizer used in the laboratory is electrical stone crystals or quinin crystals. Polarized sunglasses generally have a certain degree of flexibility, so the material of the polarizing layer is mostly a polyethylene glycol (PVA) stretching membrane and acetate cellulose membrane (TAC). Planting composite materials.

Second, the principle of polarizing sunglasses blocking glare

The glare is often formed by the reflection of sunlight on the ground, so we need to clear the polarization of the reflected light first. Generally speaking, the behavior of light on the interface is divided into reflection and refraction, and the distribution ratio of energy between them is related to the incident angle. As a fluctuation, in addition to energy distribution on the interface, the behavior of light should also consider the polarization changes of light.

Natural light can be expressed as a vibration distribution perpendicular to paper and along paper surfaces, which are represented by S and P, respectively. S and P are taken from the word heads of the word Senkrecht (vertical) and Parallel (parallel). According to the electromagnetic theory, it can be derived that after the reflection of the natural light meridian interface, the vibration component (P component) along the paper surface will weaken, so natural light becomes part of the polarized light (Figure 4).

Figure 4: The distribution of vibration after natural light reflection and refraction. Among them, dots indicate the vibration direction vertically on the paper, that is, the S component, the orange line segment indicates the vibration direction along the paper, that is, the P component. Picture source: Wang Xiaojie

Knowing the principle above, let's take a look at how the polarized sunglasses block the glare.

When the sun is reflected on the pavement, the light vibration of the reflected light is mainly S, that is, the vibration component parallel to the pavement is strong, and the vibration component perpendicular to the pavement is weak. To cover up the reflected light, the light should be blocked in the light of the polarization direction parallel to the pavement. Therefore, the transparent direction of the polarizer should be vertical, so that most of the reflected light will be filtered out, which weakens the glare (Figure 5). The above is the basic principle of polarizing sunglasses to prevent glare. Figure 5: Polarized mirror work principle

Picture source: Wang Xiaojie

Third, judge the effect of polarized sunglasses

So, how to judge whether our sunglasses are polarized sunglasses?

According to the above introduction, we know that when the natural light passes through polarizers, only the component of the vibration direction of the polarizer is left (Figure 6).

Figure 6: The distribution of vibration after natural light through polarizers

(Picture source: Wang Xiaojie)

If the two polarizers are placed in parallel along the direction of its transparency, the light can still pass through the second polarizer after the first polarizer. Figure 7A). If the second polarizer is rotated for 90 °, the light cannot pass the second polarizer after the first polarizer, and it will look dark (Figure 7B). Using this principle, we can determine whether the polarized sunglasses are polarized mirrors.

Figure 7: The light transmittance of the light in parallel and vertical vibration tablets of the light is parallel to the transparency direction of the two polarizers; (Picture source: Wang Xiaojie)

If there is no polarizer on hand, we can use the liquid crystal display (LCD). The liquid crystal display uses the light adjustment characteristics of LCD and polarizer to control the brightness of the display screen, so the light from the LCD screen is polarized light. We can take sunglasses in front of our eyes to observe the LCD screen and turn sunglasses at the same time. If you can observe the light of light → dark → bright → dark changes, then this sunglasses are polarized sunglasses (Figure 8). Strictly speaking, the angle difference between the darkest and the brightest when the light is 90 ° (Figure 7).

Figure 8 (GIF): With the help of LCD to detect polarized sunglasses

Picture source: Wang Xiaojie

Further, we can also use LCD to judge the quality of polarized sunglasses. We call the darkest and brightest light ratio as the light ratio. The lighting ratio is an important parameter for measuring the quality of polarization devices. The smaller the light is, the polarization of polarized light produced by polarization devices. The light -emitting ratio of artificial partial vibration tablets is about 1: 1000.

From the introduction above, it can be known that polarizing sunglasses can effectively weaken the glare caused by various factors such as reflexes due to increased polarizing chin layer (polarizing layer). At the same time, the polarized sunglasses often increase the ultraviolet filter layer and absorbing layer, which can block harmful ultraviolet light to a certain extent to achieve effective protection for human eyes. Ordinary sunglasses only have the effect of weakening light and cannot effectively block harmful light such as glare.

Fourth, other applications of polarizers [2]

In addition to the polarizing sunglasses, the polarizer also has many other applications in life. For example, for photography enthusiasts, polarizing mirrors are one of the most familiar tools. When shooting the surface of the water or glass room, it is often due to the reflection of light on the water or glass, which produces a strong reflection and affects the quality of shooting. At this time, add a polarizer to block the reflex light in front of the camera, which can clearly observe the scenery in the water or behind the glass (Figure 9).

Figure 9: Using polarized mirror to shoot indoor scenery (a) without polarizing mirror; (b) plus polarizer. Picture source: Wang Xiaojie

When you want to observe the solar eclipse, in order to protect the damage of the eyes from the sun, the glasses made of double -layer polarizers can reduce the strength of the sun, so that it can be observed (Figure 10).

Figure 10: Holding special glasses during solar eclipse

Photo source: Light Popular Popularity Square/Veer

In addition, the 3D movies we usually watch use the polarization characteristics of light. When shooting 3D movies, you need to take two pictures of the same object with two cameras, and then polarize the two pictures. The 3D glasses made of polarizers are perpendicular to each other (Figure 11). When watching a movie, each lens allows only the picture with the same vibration direction to enter the human eye, and the picture that the two eyes are seen are different to form a outerized image.

Figure 11: Polarized 3D glasses