Why are these sunglasses glare-proof?

As office workers, driving is our common way of traveling. During the day, the sunlight is strong, and many people will choose to use sun visors to block the sunlight. Not only is it troublesome to use, but it also blocks the line of sight. However, it is not good not to block it. Driving in the sun for a long time is prone to various eye diseases. In addition, the strong light makes the eyes uncomfortable and may also cause dangerous driving.

Therefore, "sunglasses" have become a "must-have" for drivers. However, ordinary sunglasses can protect against ultraviolet rays and blue light, but they are helpless against glare. Light reflected from horizontal surfaces such as roads, water, snow or front windows will produce glare, which will seriously affect the driver's vision, and short-term blind spots can easily cause traffic accidents.

Therefore, polarized sunglasses have become a better choice for drivers, which can effectively reduce glare. However, there is one scenario that needs to be emphasized: in low light conditions, such as at night, especially when driving, wearing any type of sunglasses will further weaken the light and affect the field of vision, so it is not recommended to wear them at night.

We have mentioned before that polarized sunglasses are more magical than ordinary sunglasses? Why is that? Today, let us explore the secrets of polarized sunglasses together.

From the literal meaning, the difference between polarized sunglasses and ordinary sunglasses is mainly reflected in the word "polarized". What is the mystery behind these two simple words? To understand the secret of polarized sunglasses, we need to first understand the core structure of polarized sunglasses - polarizer.

1. The core structure of polarized sunglasses

——Polarizer [1]

Modern optical theory tells us that light is an electromagnetic wave. Since it is a wave, is it a transverse wave or a longitudinal wave ?

In 1809, French physicist Etienne Louis Malus discovered the polarization of light and proved that light is a transverse wave, which means that the vibration direction of the electric vector of the light wave is perpendicular to the propagation direction of light. The electric vector in the two-dimensional space perpendicular to the propagation direction has various vibration states, which are called the polarization state or polarization structure of light.

According to the changing rules of the size and direction of the electric vector of light waves, light can be divided into natural light (Figure 2a), linearly polarized light (Figure 2b), partially polarized light (Figure 2c), etc. Common light sources in our lives are natural light, such as sunlight, lamplight, etc. The vibration direction of the emitted light is chaotic. Macroscopically, it contains vibrations in all directions, which are converted into linearly polarized light or partially polarized light through interface reflection. The vibration direction of linearly polarized light is usually very fixed, containing only a single vibration direction. Partially polarized light is light between natural light and linearly polarized light, and the vibration intensity in different directions is different. After understanding the different states of light, we can apply this theory to analyze the specific process of how polarized sunglasses work.

Figure 2: Several common light vibration distributions: (a) natural light; (b) linearly polarized light; (c) partially polarized light.

Image source: Wang Xiaojie

The transverse wave characteristics of light are similar to those of mechanical waves, and we can use mechanical waves to simulate the polarization of light. The reason why polarized sunglasses can filter glare is that polarized sunglasses absorb less light in a certain direction, so more light can pass through (Figure 3a); while vibrations in other directions are absorbed more, so very little light passes through (Figure 3b). This property is called dichroism, and optical components with dichroism are called polarizers, in which the vibration direction that can transmit light is called the transmission direction.

Figure 3: Using mechanical waves to simulate the polarization of light (a) The vibration direction of the wave is parallel to the slit; (b) The vibration direction of the wave is perpendicular to the slit. Image source: Wang Xiaojie

Generally speaking, the polarizers used in laboratories are tourmaline crystals or quinine sulfate crystals. Polarized sunglasses generally have a certain degree of flexibility, so the material of the polarizing layer is mostly a composite material made of polyvinyl alcohol (PVA) stretched film and cellulose acetate film (TAC) through multiple compounding, stretching, coating and other processes.

2. The principle of polarized sunglasses blocking glare

Glare is often caused by sunlight reflecting off the ground, so we need to first understand the polarization state of the reflected light. Generally speaking, the behavior of light on an interface is divided into reflection and refraction, and the energy distribution ratio between them is related to the angle of incidence. As a wave, the behavior of light on an interface should consider issues such as the change in the polarization state of light in addition to energy distribution.

Natural light can be expressed as vibration distribution perpendicular to the paper surface and along the paper surface, represented by s and p respectively. s and p are taken from the initials of the German words senkrecht (perpendicular) and parallel (parallel). According to electromagnetic theory, it can be deduced that after natural light is reflected by the interface, the vibration component along the paper surface (p component) will become weaker, so natural light becomes partially polarized light (Figure 4).

Figure 4: Distribution of vibrations of natural light after reflection and refraction. The dots represent vibrations perpendicular to the paper, i.e., the s component, and the orange lines represent vibrations along the paper, i.e., the p component. Image source: Wang Xiaojie

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

When sunlight shines on the road and reflects, the light vibration of the reflected light is mainly s component, that is, the vibration component parallel to the road surface is stronger, while the vibration component perpendicular to the road surface is weaker. To block the reflected light, the light whose polarization direction is parallel to the road surface should be blocked. Therefore, the transmission direction of the polarizer should be vertical, so that most of the reflected light will be filtered out, thereby reducing glare (Figure 5). The above is the basic principle of polarized sunglasses to prevent glare.

Figure 5: Working principle of polarizing filter. Image source: Wang Xiaojie

3. Determine the effect of polarized sunglasses

So, how do we determine whether our sunglasses are polarized sunglasses ?

According to the above introduction, we know that when natural light passes through the polarizer, only the component along the transmission direction of the polarizer remains (Figure 6).

Figure 6: Distribution of vibrations of natural light after passing through a polarizer. Image source: Wang Xiaojie

If two polarizers are placed parallel to their transmission direction, light can pass through the second polarizer smoothly after passing through the first polarizer, so the polarizer still looks transparent, but the light intensity is slightly weaker (Figure 7a). If the second polarizer is rotated 90°, the light cannot pass through the second polarizer after passing through the first polarizer, and it will look pitch black (Figure 7b). Using this principle, we can determine whether polarized sunglasses are polarized.

Figure 7: Light transmission when light passes through two polarizers with parallel and perpendicular transmission directions respectively: (a) The transmission directions of the two polarizers are parallel; (b) The transmission directions of the two polarizers are perpendicular.

Image source: Wang Xiaojie

If there is no polarizing filter at hand, we can use a liquid crystal display (LCD). The LCD screen uses the light modulation characteristics of liquid crystal combined with a polarizer to control the brightness of the screen, so the light emitted by the LCD screen is polarized light. We can put sunglasses in front of our eyes to observe the LCD screen and turn the sunglasses at the same time. If we can observe that the light changes from bright to dark to bright to dark , then this pair of sunglasses is polarized sunglasses (Figure 8). Strictly speaking, the angle difference between the sunglasses when the light is the darkest and the brightest is 90° (Figure 7).

Figure 8 (gif): Using LCD to detect polarized sunglasses. Image source: Wang Xiaojie

Furthermore, we can also use LCD to judge the quality of polarized sunglasses. We call the ratio of light intensity in the darkest and brightest states the extinction ratio. The extinction ratio is an important parameter to measure the quality of polarized devices. The smaller the extinction ratio, the higher the degree of polarization of polarized light produced by the polarized device. The extinction ratio of general artificial polarizers is about 1:1000.

From the above introduction, we can know that polarized sunglasses can effectively reduce glare caused by various factors such as reflection because of the addition of a polarizing layer (polarizing layer). At the same time, polarized sunglasses often have an additional UV filter layer and absorption layer, which can block harmful ultraviolet light to a certain extent and effectively protect the human eye. Ordinary sunglasses can only reduce the light intensity and cannot effectively block harmful light such as glare.

IV. Other applications of polarizers [2]

In addition to being used as polarized sunglasses, polarized filters have many other applications in life. For example, for photography enthusiasts, polarized filters are one of the most familiar tools. When shooting scenes on the water surface or in a glass room, the reflection of light on the water surface or glass often produces strong reflections, affecting the shooting quality. At this time, adding a polarized filter in front of the lens to block the reflected light can enable people to clearly observe the scenes in the water or behind the glass (Figure 9).

Figure 9: Using a polarizing filter to photograph indoor scenes (a) without a polarizing filter; (b) with a polarizing filter. Image source: Wang Xiaojie

When you want to observe a solar eclipse, in order to protect your eyes from sunlight damage, glasses made of double-layer polarized films can reduce the intensity of sunlight, so that you can observe it. In addition, the 3D movies we usually watch also use the polarization characteristics of light. When shooting a 3D movie, you need to use two cameras to simultaneously shoot two images of the same object, and then polarize the two images separately. The 3D glasses made of polarized films have mutually perpendicular transmission directions (Figure 10). When watching a movie, each lens only allows the images with the same polarization direction as its transmission direction to enter the human eye, and the two eyes see different images, thus forming a three-dimensional image.

After reading the above introduction, what other phenomena in life can you think of that utilize the principle of polarization of light?

References:

[1]. Zhao Kaihua. New Concept Physics Course in Optics[M]. Beijing: Higher Education Press, 2004.

[2]. Song Feng. Liberal Arts Physics - Physics in Life[M]. Beijing: Science Press, 2013.

Author: Wang Xiaojie, School of Physics, Nankai University, Nankai University Physics Popularization Base

Review ｜ Gao HuiHuazhong University of Science and Technology

Source: China Optics

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