📚 Polarization | 偏振
Polarization is a characteristic exclusive to transverse waves, describing the orientation of oscillations in a plane perpendicular to the direction of energy propagation. In the context of IB Physics, polarization reveals the electromagnetic nature of light and underpins many optical technologies.
偏振是横波独有的特性，描述振动在与能量传播方向垂直的平面内的取向。在 IB 物理中，偏振不仅揭示了光的电磁本质，也是许多光学技术的基础。
1. Transverse Nature of Waves and Polarization | 波的横波性质与偏振
A wave is said to be polarized when its oscillations occur in a single well-defined plane. Only transverse waves can be polarized because their vibrations are perpendicular to the direction of travel; longitudinal waves, in which particles vibrate parallel to the wave direction, cannot exhibit polarization.
当波的振动局限于一个确定平面内时，我们就说该波被偏振了。只有横波才能发生偏振，因为横波的振动方向与传播方向垂直；而纵波的质点振动平行于传播方向，无法展现偏振现象。
Light is an electromagnetic wave in which the electric and magnetic fields oscillate perpendicular to each other and to the direction of propagation. The plane of polarization is conventionally defined by the direction of the electric field vector.
光是一种电磁波，其电场和磁场彼此垂直且都垂直于传播方向。偏振面通常由电场矢量的方向来定义。
The fact that light can be polarized was historically a crucial proof that light is a transverse wave, ruling out purely longitudinal models.
历史上，光可以偏振这一事实是证明光为横波的关键证据，排除了纯纵波模型。
2. Unpolarized and Linearly Polarized Light | 非偏振光与线偏振光
Natural light sources such as the Sun and incandescent bulbs emit unpolarized light. In unpolarized light, the electric field oscillates in random directions perpendicular to the ray, constantly changing over time.
天然光源（如太阳和白炽灯）发出的是非偏振光。在非偏振光中，电场在与光线垂直的平面内随机方向振荡，且随时间不断变化。
Linearly polarized light, also called plane-polarized light, has its electric field oscillating along a single fixed direction. This can be thought of as selecting just one component of the many random oscillations present in unpolarized light.
线偏振光（也叫平面偏振光）的电场沿单一固定方向振荡。这可以理解为从非偏振光中众多随机振荡中选出了一个分量。
In diagrams, unpolarized light is represented by a combination of arrows and dots, while linearly polarized light is shown with a single double-headed arrow indicating the direction of the electric field.
在示意图中，非偏振光常用箭头和圆点的组合表示，而线偏振光则用单个双箭头标明电场的方向。
3. Polarizing Filters and Malus's Law | 偏振片与马吕斯定律
A polarizing filter (polarizer) acts as a slit that only allows the component of the electric field parallel to its transmission axis to pass through. When unpolarized light strikes an ideal polarizer, the transmitted intensity is reduced by exactly one-half: I₁ = I₀ / 2, where I₀ is the incident intensity.
偏振片（起偏器）的作用类似于一个狭缝，只允许与其透射轴平行的电场分量通过。当非偏振光射入理想偏振片时，透射光强度恰好降至原来的二分之一：I₁ = I₀ / 2，其中 I₀ 为入射光强。
If this linearly polarized light then encounters a second polarizer (called an analyzer), the transmitted intensity obeys Malus's law. The law is expressed as:
如果这束线偏振光接着遇到第二个偏振片（称为检偏器），其透射强度将遵循马吕斯定律。该定律表达为：
I = I₀ cos² θ
When θ = 0°, all light is transmitted; when θ = 90°, no light passes through (extinction).
式中 I₀ 是入射到检偏器的线偏振光强度，θ 是两个偏振片透射轴之间的夹角。当 θ = 0° 时，光全部透过；当 θ = 90° 时，无光通过（消光）。
Malus's law can be verified experimentally by rotating one polarizer relative to the other and measuring the intensity with a light sensor. The cos² relationship confirms the transverse wave model.
马吕斯定律可通过旋转一个偏振片并利用光传感器测量光强来实验验证。cos² 的关系证实了横波模型。
4. Experimental Demonstration of Polarization | 偏振的实验演示
A classic demonstration uses two polarizing sheets. With their axes parallel, maximum brightness is observed; rotating one sheet causes the transmitted light to dim gradually until it vanishes completely at the crossed position (90°).
经典的演示使用两片偏振片。当它们的透射轴平行时，观察到最大亮度；旋转其中一片，透射光逐渐变暗，直到在正交位置（90°）完全消失。
At IB level, microwave polarization experiments are also common. A microwave transmitter emits vertically polarized waves, and a receiving aerial detects the signal only when aligned with the polarization direction. A metal grid with parallel bars can act as a polarizer for microwaves: when the bars are parallel to the polarization, the waves are reflected or absorbed; when perpendicular, the waves pass through with little attenuation.
在 IB 层面上，微波偏振实验也很常见。微波发射器发出垂直偏振的波，接收天线只有在与偏振方向一致时才能检测到信号。平行金属棒的栅格可作为微波的偏振器件：当栅条平行于偏振方向时，波被反射或吸收；当栅条垂直时，波几乎无衰减地通过。
These experiments show that the intensity variation and extinction are direct consequences of the directionality of transverse vibrations.
这些实验表明，强度的变化和消光现象是横向振动具有方向性的直接结果。
5. Polarization by Reflection: Brewster's Angle | 反射偏振：布儒斯特角
When unpolarized light reflects off a dielectric surface (such as water or glass), the reflected ray becomes partially polarized. At a specific angle of incidence, called the Brewster angle θB, the reflected light is completely linearly polarized with its electric field oscillating parallel to the surface (i.e., perpendicular to the plane of incidence).
当非偏振光从介质表面（如水或玻璃）反射时，反射光会变成部分偏振光。在某个特定的入射角——布儒斯特角 θB 下，反射光变为完全线偏振光，且电场振动平行于界面（即垂直于入射面）。
The Brewster angle is given by:
tan θB = n
rays are exactly 90° apart. For water (n ≈ 1.33), θB ≈ 53°; for glass (n ≈ 1.5), θB ≈ 56°.
其中 n 是介质的折射率。在这个角度下，反射光线与折射光线恰好成 90° 夹角。对于水（n ≈ 1.33），θB ≈ 53°；对于玻璃（n ≈ 1.5），θB ≈ 56°。
Polarizing sunglasses take advantage of this effect. They contain a polarizing filter oriented to block the horizontally polarized light reflected from horizontal surfaces, such as roads and water, thereby reducing glare.
偏振太阳镜利用了这一效应。镜片中的偏振片方向经过设计，能够阻挡从水平表面（如路面和水面）反射的水平偏振光，从而减少眩光。
6. Polarization by Scattering | 散射偏振
Light scattered by the atmosphere (Rayleigh scattering) also becomes partially polarized. When sunlight interacts with small molecules in the air, the electric field components perpendicular to the scattering plane oscillate most effectively, causing the light observed at 90° to the direction of the Sun to be strongly polarized.
大气散射（瑞利散射）也会导致光发生部分偏振。当阳光与空气中的小分子相互作用时，垂直于散射平面的电场分量振荡最有效，这使得在与太阳方向成 90° 位置观察到的光具有很强的偏振性。
The blue sky overhead is often partially polarized, and the degree of polarization depends on the angle from the Sun. Many insects, such as bees, can detect the polarization pattern of the sky, using it as a compass for navigation even when the Sun is hidden.
头顶的蓝天常常是部分偏振的，偏振程度取决于与太阳的夹角。许多昆虫，如蜜蜂，能感知天空的偏振图样，将其作为导航的“罗盘”，即使在太阳被遮挡的情况下也能使用。
In photography, a polarizing filter can be rotated to darken the blue sky and enhance cloud contrast by suppressing scattered polarized light.
在摄影中，旋转偏振滤镜可以抑制散射的偏振光，使蓝天变暗并增强云层的对比度。
7. Optical Activity and Stress Polarization | 旋光性与应力偏振
Certain materials, such as sugar solution or quartz, can rotate the plane of polarization of linearly polarized light passing through them. This property is called optical activity and is used in saccharimeters to measure sugar concentration.
某些材料（如糖溶液或石英）能使穿过其中的线偏振光的偏振面发生旋转。这一性质称为旋光性，可用于糖量计中测量糖的浓度。
Some transparent materials become birefringent when subjected to mechanical stress, meaning they split incident light into two polarized components. When viewed between crossed polarizers, stressed regions display colourful fringe patterns that reveal internal stress distributions.
一些透明材料在受到机械应力时会变成双折射材料，即它们将入射光分为两束偏振分量。在正交偏振片之间观察时，应力区域会显示出彩色条纹图案，显示出内部应力分布。
Photoelasticity is a practical application of stress polarization: engineers build transparent scale models of mechanical parts and examine them under polarized light to identify stress concentrations before manufacturing.
光弹性方法是应力偏振的实际应用：工程师制作机械零件的透明比例模型，在偏振光下检查，从而在制造前识别应力集中区域。
8. Circular and Elliptical Polarization | 圆偏振与椭圆偏振
In addition to linear polarization, light can be circularly or elliptically polarized. Circularly polarized light consists of two perpendicular electric field components of equal amplitude that are 90° out of phase; the resultant electric field vector rotates in a helix as the wave propagates.
除了线偏振，光还可以是圆偏振或椭圆偏振。圆偏振光由两个振幅相等且相位差 90° 的垂直电场分量组成；随着波的传播，合成电场矢量以螺旋方式旋转。
A quarter-wave plate is an optical device that converts linearly polarized light into circularly polarized light when the incident polarization is at 45° to its fast and slow axes. This principle is used in 3D cinema glasses and some anti-glare screens.
四分之一波片是一种光学器件，当入射的线偏振方向与其快轴和慢轴成 45° 角时，可将其转化为圆偏振光。这一原理用于 3D 电影眼镜和一些防眩屏幕。
Elliptical polarization is a more general case where the two components differ in amplitude and have a phase difference other than 0° or 90°. Both circular and elliptical polarization illustrate the full vector nature of electromagnetic waves.
椭圆偏振是更一般的情况，两个分量振幅不同且相位差不为 0° 或 90°。圆偏振和椭圆偏振共同体现了电磁波的完全矢量特性。
9. Applications: Polarizing Sunglasses and Photography | 应用：偏振太阳镜与摄影
Polarizing sunglasses exploit Brewster's law to eliminate horizontally polarized reflected glare. By orienting the transmission axis vertically, the filter blocks the intense horizontal component reflected from roads, water, and wet surfaces, leading to safer driving and clearer vision.
偏振太阳镜利用布儒斯特定律消除水平偏振的反射眩光。通过将透射轴垂直放置，滤光片阻挡了从路面、水面和湿滑表面反射的强烈水平分量，使驾驶更安全、视觉更清晰。
Photographers use circular polarizing filters to manage reflections from glass and water, to saturate colours by reducing specular highlights, and to darken the sky. Adjustable polarizing filters give creative control over the appearance of non-metallic surfaces.
摄影师使用圆形偏振滤镜来管控玻璃和水面的反射，通过减少镜面高光使色彩更饱和，并压暗天空。可调的偏振滤镜可对非金属表面的外观进行创意控制。
10. Applications: Liquid Crystal Displays (LCDs) | 应用：液晶显示器
Modern LCD screens rely fundamentally on polarization control. A liquid crystal layer is sandwiched between two crossed polarizers. When no voltage is applied, the liquid crystal twists the plane of polarization of the light by 90°, allowing it to pass through the second polarizer, and the pixel appears bright.
现代液晶显示器从根本上依赖于偏振控制。液晶层被夹在两片正交偏振片之间。当未施加电压时，液晶将光的偏振面旋转 90°，使其能够透过第二片偏振片，像素呈亮态。
When a voltage is applied, the liquid crystal molecules realign, no longer twisting the polarization direction. The light is then blocked by the second polarizer, and the pixel appears dark. By controlling the voltage across each subpixel, full-colour images are generated.
当施加电压时，液晶分子重新排列，不再扭曲偏振方向。此时光被第二片偏振片阻挡，像素呈暗态。通过控制每个子像素的电压，便可生成全彩图像。
This precise manipulation of polarized light demonstrates how fundamental physics underpins everyday digital technology.
这种对偏振光的精确操控展示了基础物理学如何支撑日常数字技术。
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