Polarized Light
Polarized light is a term in optics. Light is an electromagnetic wave, and electromagnetic waves are transverse waves. The plane formed by the direction of vibration and the direction of propagation of light waves is called the plane of vibration. The vibration plane of light is limited to a certain fixed direction, called plane-polarized light or linearly polarized light.
Definition
The asymmetry of the vibration direction with respect to the propagation direction is called polarization, which is the most obvious sign that distinguishes transverse waves from other longitudinal waves, as only transverse waves exhibit polarization phenomena. Light waves are electromagnetic waves; therefore, the direction of propagation of light waves is the direction of propagation of electromagnetic waves. The electric and magnetic vibration vectors E and H of light waves are perpendicular to the propagation velocity v; thus, light waves are transverse waves and possess polarization. Light with polarization is called polarized light.
Classification
Polarized light refers to light waves whose vibration direction of the light vector remains unchanged or changes in a certain regular manner. According to their properties, polarized light can be further divided into several types: plane-polarized light (linearly polarized light), circularly polarized light, elliptically polarized light, and partially polarized light. If the vibration direction of the electric vector of the light wave is confined to a certain plane, this polarized light is called plane-polarized light, or linearly polarized light because the direction of vibration remains a straight line during propagation. If the electric vector of the light wave changes regularly with time, with the end trajectory of the electric vector forming a circle or ellipse perpendicular to the direction of propagation, it is called circularly polarized light or elliptically polarized light. If the vibration of the light wave's electric vector is predominantly in a certain direction during propagation, this polarized light is called partially polarized light.
Generation Mechanisms
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Reflection, Multiple Refraction, Birefringence, and Selective Absorption: Plane-polarized light can be obtained through methods such as reflection, multiple refraction, birefringence, and selective absorption. Polarizers with selective absorption can be used to produce plane-polarized light.
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Polarizing Film: Polarizing films are thin films made by arranging microcrystals with strong selective absorption in a transparent gel layer using special methods. They allow light with a certain polarization direction (this direction is called the polarization direction) to pass through and absorb light perpendicular to it, exhibiting dichroism. Therefore, when natural light passes through a polarizing film, the transmitted light becomes essentially plane-polarized light. Since polarizing films are easy to manufacture, they are widely used as polarizers.
Characteristics
Transverse waves have a property that their vibrations are polarized. In the plane perpendicular to the direction of propagation, they can vibrate in any direction. The direction of vibration of the electric field of the light wave is generally considered as the direction of light vibration. If a beam of light vibrates in the same direction, it is called polarized light, or more strictly, completely polarized light. Ordinary natural light vibrates uniformly in all directions, resulting in non-polarized light. However, the glare formed by the reflection of smooth non-metallic surfaces at a certain angle (called the Brewster angle, related to the refractive index of the material) is polarized light. If this angle is deviated from, partially unpolarized light will be mixed in the polarized light. This light is called partially polarized light and has a certain degree of polarization. The greater the deviation angle, the fewer components of polarized light, eventually becoming non-polarized light.
Applications
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Car Headlights: Car headlights are equipped with polarizers on the front window and the glass cover of the headlights. These polarizers are oriented in the same direction at a 45-degree angle to the horizontal plane. Therefore, when the driver looks through the front window, they can only see the light emitted by their own headlights, not the light from the oncoming headlights. This allows cars to drive safely at night without dimming the headlights or slowing down.
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3D Movies: When shooting 3D movies, two cameras are used, equivalent to a person's two eyes. They simultaneously capture two images of the same object, and when projected, both images are displayed on the screen simultaneously. By ensuring that one eye of the viewer can only see one of the images, a sense of depth can be achieved. Therefore, during projection, polarizers are placed on each lens of the two projectors, with the polarization directions perpendicular to each other. Viewers wear glasses made of polarizing film, with the polarization direction of the left eye matching that of the left projector, and the polarization direction of the right eye matching that of the right projector. In this way, each eye observes one image on the screen, creating a stereoscopic image in the viewer's mind.
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Camera Lenses: When natural light reflects off surfaces such as glass, water surfaces, wooden tables, etc., both the reflected and refracted light are polarized light, and the degree of polarization also changes with the angle of incidence. When shooting smooth objects such as glassware, water surfaces, display cabinets, painted surfaces, plastic surfaces, etc., glare or reflections often occur due to interference of reflected light waves. By adding a polarizing filter during shooting and appropriately rotating the polarizing filter so that its transmission direction is perpendicular to the transmission direction of the reflected light, the reflected light can be weakened, making the image behind water or glass clear.
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Measurement of Physical Quantities: After polarized light passes through certain media, its vibration direction relative to the original vibration direction undergoes a certain angle of rotation. This angle of rotation is called optical rotation, and it is related to factors such as the concentration, length, and refractive index of the medium. By measuring the magnitude of optical rotation, changes in physical quantities related to the medium can be determined.
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Biological and Physiological Functions: Human eyes cannot distinguish the polarization state of light, but the eyes of some insects are very sensitive to polarization. For example, bees have five eyes, three simple eyes, and two compound eyes, each compound eye containing 6300 small eyes. These small eyes can determine the direction of the sun based on polarized light, and then use the sun as a directional marker to determine direction. Therefore, bees can accurately guide their counterparts to the flower patches they find. Similarly, in the desert, ants can navigate using skylight polarization, thus not getting lost.
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LCD Screens: LCD technology fills 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 north-south, then those on the other plane are arranged east-west, and the molecules between the two planes are forced into a state of 90-degree twist. Since light propagates along the direction of molecular alignment, light passing through the liquid crystal is also twisted by 90 degrees. However, when a voltage is applied to the liquid crystal, the molecules will realign vertically, allowing the light to pass straight through without any twist.
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