Birefringence

Calcite

方解石

The mineral calcite, also known as Iceland spar, is a widely used material in optics because of its birefringence. Its birefringence is so large that a calcite crystal placed over a dot on a page will reveal two distinct images of the dot. One image will remain fixed as the crystal is rotated, and that ray through the crystal is called the "ordinary ray" since it behaves just as a ray through glass. However, the other image will rotate with the crystal, tracing out a small circle around the ordinary image. This ray is called the "extraordinary ray".

碳酸钙矿物，也称为冰洲石，因其双折射特性而被广泛用于光学领域。它的双折射非常显著，当一个冰洲石晶体放置在书页上的一个点上时，会显示出该点的两个不同的影像。其中一个影像在晶体旋转时保持固定，通过晶体的这条光线被称为‘寻常光线’，因为它在晶体中的行为就像通过玻璃的光线。然而，另一个影像会随着晶体的旋转而旋转，围绕寻常影像画出一个小圆圈。这条光线被称为‘非寻常光线’。

The indices of refraction for the o- and e-rays are 1.6584 and 1.4864 respectively. This gives total internal reflection critical angles of 37.08° for the o- and 42.28° for the e-rays when in contact with air. This means that for any angle between these two values, the o-ray will be totally reflected but the e-ray will be partially transmitted. This gives linear polarization since only the e-ray emerges.

o-和e光的折射率分别为1.6584和1.4864。这导致当与空气接触时，o光的全内反射临界角为37.08°，而e光为42.28°。这意味着对于介于这两个值之间的任何角度，o光将完全内反射，而e光将部分透射。这会导致线性偏振，因为只有e光会透出。

方解石用于制作偏振棱镜，如尼柯尔棱镜、格兰-傅科棱镜和沃尔拉斯顿棱镜。

A simple demonstration of the large birefringence of calcite is to put a dot on a piece of paper and put the calcite crystal over it. You see two distinct dots. By putting a piece of polaroid over the crystal and rotating it, you can show that the two images of the dot are made up of light polarized at 90° with respect to each other. Rotating the polaroid will show one dot, then both in transition, and then just the second dot as you reach 90°.

一个简单的双折射现象的演示是将一个点放在一张纸上，然后将方解石晶体放在其上。你会看到两个明显的点。通过在晶体上放置一块偏振片并旋转它，可以显示这两个点是由相互垂直偏振的光组成的。旋转偏振片时，你会看到一个点，然后是两个点逐渐过渡，最后当旋转到90°时，只显示第二个点。

The video segment below shows the successive disappearance of the two dots as a sheet of polaroid is rotated over them.

下面的视频片段显示，当偏振片依次覆盖在两个点上时，这两个点依次消失。