我们先从两个基础概念说起：反射与折射。

Let’s start with two fundamental concepts: reflection and refraction.

当光到达两种介质的交界面时，既可能发生反射也可能产生折射。反射是指光线返回原介质的现象，其反射角始终与入射角相等。

When light reaches the interface between two media, it may undergo reflection or refraction. Reflection is the phenomenon where light rays return to the original medium, and the angle of reflection is always equal to the angle of incidence.

折射则是光线进入新介质后因折射率差异导致传播方向改变的过程。

Refraction is the process where light enters a new medium and changes its direction of propagation due to a difference in refractive indices.

光的反射和折射都发生在入射面内，也就是入射光线和界面法线构成的平面。

Both reflection and refraction of light occur within the plane of incidence, which is the plane defined by the incident ray and the normal to the interface.

折射角、入射角正弦之比等于相对折射率：

The ratio of the sines of the angle of refraction to the angle of incidence equals the relative refractive index:

也就是折射定律，它的另外一个名字我们可能听得比较少，Snell定律或Snell-Descartes 定律（斯涅尔-笛卡尔定律）。

This is the Law of Refraction, by another name you might be less familiar with: Snell’s law or Snell-Descartes law.

生活中的现象（所以河水实际上比看起来的更深）：

Phenomenon in daily life (so the river is actually deeper than it appears):

那什么是双折射呢？

So, what is birefringence?

一束入射到介质中的光经折射后变为两束光，称为双折射。

When a beam of light entering a medium is refracted into two beams, it is called birefringence.

需要特别说明的是，仅各向异性晶体具备这种特性——这类晶体的原子排列具有方向选择性，导致不同偏振方向的光线产生相异折射率。

It must be specifically noted that only anisotropic crystals possess this property – the atomic arrangement in this type of crystal has directional selectivity, resulting in different refractive indices for light with different polarization directions.

下图是一个机械振子模型，可以帮我们简单理解各向同性和各向异性。

The figure below is a mechanical oscillator model, which can help us simply understand isotropy and anisotropy.

如上图，各向异性模型中z轴特性不一样。

As shown above, the properties along the z-axis are different in the anisotropic model.

以具有双折射性质的方解石为例：

Taking calcite, which has birefringent properties, as an example:

折射后的两束光都是线偏振光。一束遵循折射定律，称为寻常光（o光ordinary ray）,一束不遵循折射定律，称为非常光（e光，extraordinary ray）。

The two refracted beams are both linearly polarized light. One beam follows the law of refraction and is called the ordinary ray (o-ray); one beam does not follow the law of refraction and is called the extraordinary ray (e-ray).

利用这种特性可以制作很多有用的光学器件，我们简单介绍两种。

This property can be used to make many useful optical devices; we will briefly introduce two.

比如，Wallaston棱镜，用两块方解石直角三棱镜构成：

For example, the Wollaston prism, constructed from two calcite right-angle prisms:

光垂直入射第一直角棱镜（光轴垂直纸面）时，o/e光未分离但存在速差（方解石中e光速快）；进入第二棱镜（光轴竖直）后o/e光互换角色，最终输出两束偏振正交且具夹角的线偏振光。

When light is incident perpendicularly on the first right-angle prism (optical axis perpendicular to the page), the o/e rays are not separated but have a velocity difference (e-ray is faster in calcite); after entering the second prism (optical axis vertical), the o/e rays swap roles, and finally output two linearly polarized beams that are orthogonally polarized and have an angular separation.

Babinet-Soleil 补偿器，一块固定平面平行板+两片可移动楔形板（光轴方向与固定板垂直）

Babinet-Soleil compensator, one fixed plane-parallel plate + two movable wedge plates (optical axis direction perpendicular to the fixed plate)

楔形板可以沿斜面滑动，因此有效厚度可以调节，可以用来调节光的相位延迟，实现相位补偿。

The wedge plates can slide along the incline, so the effective thickness can be adjusted; this can be used to adjust the phase retardation of light, achieving phase compensation.

hank you for reading. Suggestions for improvement are welcome if any inaccuracies are noted.

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