What is polarized light?

Polarized light, optical terminology. Light is electromagnetic wave, and electromagnetic wave is shear wave. The plane formed by the vibration direction and the light wave direction is called the vibration plane, and the vibration plane of light is limited in a certain fixed direction, which is called plane polarized light or linear polarized light.

definition

The asymmetry between vibration direction and propagation direction is called polarization, which is the most obvious sign that shear waves are different from other longitudinal waves. Only shear waves have polarization. Light waves are electromagnetic waves, so the propagation direction of light waves is the propagation direction of electromagnetic waves. The electric vibration vector E and the magnetic vibration vector H in the light wave are perpendicular to the propagation velocity V, so the light wave is shear wave and polarized. Polarized light is called polarized light.

App application

Car lamp

When the car is driving on the expressway at night, in order to avoid the glare of the headlights on both sides, drivers turn off the headlights and only turn on the small lights to slow down the speed, so as to avoid accidents. For example, both the front window glass of the cab and the headlight glass cover are equipped with polarizers, which stipulate that their polarization directions are in the same direction and make an angle of 45 degrees with the horizontal plane. Then, the driver can only see the light from his headlight from the front window, but can't see the light from the opposite headlight. In this way, when the car is driving at night, it is not necessary to turn off the lights or slow down, which can ensure safe driving.

In addition, when driving a car on a sunny day, the dazzling sunlight reflected from the glass of the road or the surrounding buildings often makes your eyes hard to open. Because light is a shear wave, these strong scattered light from the sky basically vibrates horizontally. Therefore, only a pair of polarized sunglasses that can only transmit polarized light in the vertical direction can block some scattered light.

Watch stereoscopic movies

When shooting a three-dimensional movie, two cameras are used. The lenses of two cameras are equivalent to two human eyes. They take two portraits of the same object at the same time, and both portraits are reflected on the screen at the same time. If one eye of the audience can only see one of the pictures, it can give the audience a three-dimensional sense. Therefore, during the screening, a polarizer is placed on the lens of each of the two players, and the polarization directions of the two polarizers are perpendicular to each other. Spectators wear glasses made of polarizers. The polarization direction of the left-eye polarizer is the same as that of the left-handed player, and the polarization direction of the right-eye polarizer is the same as that of the right-handed player.

camera lens

When natural light is reflected on glass, water surface, wooden desktop and other surfaces, both reflected light and refracted light are polarized light, and the degree of polarization also changes when the incident angle changes. When shooting objects with smooth surfaces, such as glassware, water, display cabinets, painted surfaces, plastic surfaces, etc. Flares or reflections often occur, which are caused by the interference of reflected light waves. If you add a polarizer when shooting, and properly rotate the polarizer to make its transmission direction perpendicular to the direction of reflected light, you can weaken the reflected light and make the image under water or behind glass clear.

Measuring related physical quantities

After polarized light passes through a medium, its vibration direction will rotate at a certain angle relative to the original vibration direction. The rotation angle is called optical rotation, which is related to the concentration, length and refractive index of the medium. By measuring the optical rotation, we can know the changes of physical quantities related to the medium.

Biological physiological function

The human eye cannot distinguish the polarization state of light, but the eyes of some insects are very sensitive to polarization. For example, a bee has five eyes, three single eyes and two compound eyes, and each compound eye contains 6300 small eyes. These small eyes can determine the direction of the sun according to the polarization of the sun, and then use the sun as a direction marker to judge the direction, so the bee can accurately guide its own kind to the flowers it finds.

Another example is in the desert, if you don't take a compass, people will get lost, but there is an ant in the desert, which can navigate through ultraviolet polarization in the sky, so it won't get lost.

liquid-crystal display

LCD technology is to pour the liquid crystal between two planes with fine grooves. The grooves on these two planes are perpendicular to each other (intersecting at 90 degrees). That is to say, if the molecules on one plane are arranged in the north-south direction and the molecules on the other plane are arranged in the east-west direction, the molecules located between the two planes are forced to be in a 90-degree twisted state. Because light propagates along the arrangement direction of molecules, it is also distorted by 90 degrees when passing through liquid crystal. However, when a voltage is applied to the liquid crystal, the molecules will rearrange vertically, so that light can be emitted directly without any twist.

LCD relies on polarizing filter (sheet) and light itself. Natural light scatters randomly in all directions. Polarization filters are actually a series of thinner and thinner parallel lines. These lines form a net, blocking all light that is not parallel to these lines. The line of polarization filter is just perpendicular to the first one, so it can completely block polarized light. Only when the lines of the two filters are completely parallel, or the light itself has been twisted to match the second polarizing filter, can the light penetrate.

LCD is composed of these two orthogonal polarization filters, so under normal circumstances, it should block all the light that tries to penetrate. However, because the twisted liquid crystal is filled between the two filters, after the light passes through the first filter, it will be twisted by liquid crystal molecules by 90 degrees and finally pass through the second filter. On the other hand, if a voltage is applied to the liquid crystal, the molecules will be rearranged and completely parallel, so that the light is no longer distorted, so it is only blocked by the second filter. In short, light is emitted without power, and light is blocked with power.