Polarization principle and application of light?

The asymmetry between the polarization vibration direction and the propagation direction of light is called polarization, which is the most obvious sign that transverse waves are different from other longitudinal waves. The phenomenon that the spatial distribution of photoelectric vector vibration of light wave loses symmetry with respect to the propagation direction of light is called polarization of light. Only shear waves can produce polarization, so the polarization of light is another example of light fluctuation. In the plane perpendicular to the propagation direction, it contains transverse vibration in all possible directions. On average, it has the same amplitude in any direction. This kind of light whose transverse vibration and propagation direction are symmetrical is called natural light (unpolarized light). All light that loses this symmetry in vibration is called polarized light.

1, linearly polarized light

In the process of light propagation, there is only one kind of vibration, and its vibration direction always stays in the same plane. This kind of light is called linearly polarized light (or plane polarized light). You can imagine this through an experiment: you tie one end of the rope to a tree in your neighbor's yard and hold the other end in your hand. Let's suppose that the rope passes between two bamboos in the fence. If you vibrate the rope up and down now, the wave generated by the rope will pass between two bamboos and pass from your hand to the tree. At this time, the fence is "transparent" to your wave. However, if you let the rope wave from side to side, the rope will touch two bamboos and the waves will not go through the fence. At this time, the fence is equivalent to a polarizer.

2. Partially polarized light

Light waves contain transverse vibrations in all possible directions, but the amplitudes in different directions are not equal, and the amplitudes have maximum and minimum values in two mutually perpendicular directions. This light is called partially polarized light. Natural light and partially polarized light are actually composed of many linearly polarized lights with different vibration directions.

When light enters the medium from air (strictly speaking, it should be vacuum), the tangent value of Brewster angle is equal to the refractive index n of the medium. Because the refractive index of medium is related to the wavelength of light, Brewster angle is also related to the wavelength of light for the same medium. According to the refractive index of optical glass 1.4- 1.9, Brewster angle is about 54-62 degrees. When the incident angle deviates from Brewster angle, the reflected light will be partially polarized light.

3. Elliptically polarized light

In the process of light propagation, the electric vector of each point in space rotates around the light, and the endpoint of the electric vector describes an elliptical trajectory, which is called elliptically polarized light. Facing the light direction, all electric vectors rotating clockwise are called right-handed elliptically polarized light, and all electric vectors rotating counterclockwise are called left-handed elliptically polarized light. The rotating electric vector in elliptically polarized light is the result of vibration synthesis of two electric vectors with the same frequency, vertical vibration direction and fixed phase difference.

4. Circularly polarized light

The light moving along a circular trajectory at the end of a rotating electric vector is called circularly polarized light, which is a special case of elliptically polarized light. After averaging during our observation period, circularly polarized light looks the same as natural light. The polarization direction of circularly polarized light changes according to a certain law, while the polarization direction of natural light changes randomly and irregularly.

Application:

1. The LCD of the electronic watch uses polarized light.

The liquid crystal box is inserted between two polarizers whose vibration transmission directions are perpendicular to each other. Transparent electrode plates are arranged above and below the liquid crystal layer in the box, and the electrode plates are carved into the shape of digital strokes. Natural light from the outside passes through the first polarizer and becomes polarized light. When this beam of light passes through the liquid crystal, if there is no voltage between the upper and lower plates, the polarization direction of the light will be rotated by 90 degrees by the liquid crystal (this property is called the optical rotation of the liquid crystal), so it can pass through the second polarizer. Below the second polarizer is a mirror, and the light is reflected back. At this point, the liquid crystal cell looks transparent. However, when there is a certain voltage between the upper and lower electrodes, the properties of the liquid crystal change, the optical rotation disappears, the light cannot pass through the second polarizer, and the area below this electrode becomes dark. If the electrodes are carved into the shape of digital strokes, numbers can be displayed in this way.

Polarizer effect

2. Add a polarizer in front of the camera lens to eliminate reflection.

When shooting objects with smooth surfaces, such as glassware, water, display cabinets, painted surfaces, plastic surfaces, etc. Flare or reflection often occurs, which is caused by the polarization of light. When shooting, adding a polarizer and properly rotating the polarizer surface can block these polarized lights, thus eliminating or weakening the reflections or bright spots on the surfaces of these smooth objects. Turn the reflector while observing through the viewfinder to observe the effect of eliminating polarized light. When it is observed that the reflection of the object disappears, the rotation of the mirror can be stopped.

3. Control the brightness of the sky during photography to darken the blue sky.

Because there is a lot of polarized light in the blue sky, you can adjust the brightness of the sky with a polarizer. After adding a polarizer, the blue sky becomes very dark, highlighting the white clouds in the blue sky. The polarizer is gray, so both black and white and color photography can be used.

Step 4 watch stereoscopic movies with polarizers

When watching a three-dimensional movie, the audience should wear a pair of special glasses, which are polarizers whose vibration transmission directions are perpendicular to each other. Three-dimensional movies are composed of two lenses, such as human eyes, shooting scenes from two different directions at the same time. During screening, two groups of films shot by two cameras are simultaneously screened by two projectors, so that two slightly different images overlap on the screen. At this time, if you look directly with your eyes, the picture you see is blurred. To see stereoscopic movies, a polarizer must be installed in front of each movie machine, and the role of the polarizer is to play the role of a polarizer. The light emitted from the two projectors becomes polarized light after passing through the polarizer. The polarization directions of the polarizers in front of the left projector and the right projector are perpendicular to each other, so the polarization directions of the two polarized lights are also perpendicular to each other. These two beams of polarized light are projected on the screen and then reflected to the audience, and the direction of polarized light remains unchanged. When the audience watches with the polarized glasses, each eye can only see the corresponding polarized light image, that is, the left eye can only see the picture reflected by the left machine, and the right eye can only see the picture reflected by the right machine, which will produce a three-dimensional effect like direct viewing. This is the principle of three-dimensional movies.

Of course, the actual projection of three-dimensional movies is to use one lens, and two groups of images are alternately printed on the same film, which requires complex devices. The propagation of light in crystals is closely related to polarization. Polarization can be used to understand the optical characteristics of crystals, manufacture optical devices for measurement, and provide technical means such as rock and mineral identification, photoelastic measurement and laser modulation.

5. Biological and physiological functions and polarized light

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.

6. The car uses a polarizer to prevent the glare of the opposite light at night.

High beam is annoying, but the polarization of light can solve this problem. We can design the car lampshade as a polarizing lens with an inclined direction of 45, so that the light emitted is regular oblique light. The driver wears a pair of night vision goggles with the same polarization direction as the lampshade. In this way, the driver can only see the light from his own car, and the vibration direction of the light from the opposite car is exactly 90 degrees with the direction of the car, so that the light from the opposite car light will not shake the driver's eyes again.

Of course, there is still a long way to go to realize this idea. First of all, the world must formulate a unified standard to specify the polarization direction of lampshades and glasses; Secondly, polarized glasses will inevitably lose some light, so the driver's vision will be affected; Moreover, car headlights's power is very high, and half of the energy is absorbed by polarized lenses, which will definitely generate a lot of heat, which is also a very big test for the work of automobile lampshades.