尼科尔棱镜1828

它是第一种偏振棱镜，由爱丁堡的威廉·尼柯耳（1770-1851）于1828年发明。

机制

尼柯耳棱镜由冰岛晶石（一种方解石）的菱面体晶体组成，该晶体相对于晶体轴以68°的角度切割，再次对角线切割，然后使用一层透明的加拿大香脂作为胶水重新连接。

非偏振光线通过晶体的侧面进入，并被方解石的双折射特性分成两条正交偏振、方向不同的光线。“普通”光线或“o光线”在方解石中的折射率为no =1.658，因为其在胶层处（折射率n=1.550）的入射角超过了界面的临界角，因此在方解石-胶界面处发生全反射。于是它带有一些折射，穿过棱镜上半部分的顶侧。“异常”光线或“e光线”在方解石晶体中的折射率较低（ne=1.486），并且不会在界面上完全反射，因为它以低于临界角撞击界面。当“e光线”穿过界面进入棱镜的下半部分时，它只会经历轻微的折射或弯曲。它最终以平面偏振光的形式离开棱镜，在离开棱镜的另一侧时经历另一次折射。假设方解石棱柱角已被适当切割，则两条出射光线具有相互垂直的偏振，但较低的光线或“e光线”，因为它再次沿原始水准方向传播，因此更常用于进一步的实验。 上光线或“o光线”的方向与其原始方向大不相同，因为它在胶水界面处单独发生全反射，并在棱镜上侧出射时发生最终折射。

使用

尼柯耳棱镜曾广泛应用于矿物学显微镜和偏振测量，术语“使用交叉尼柯耳”（缩写为“XN”）仍用于指观察放置在正交取向偏振器之间的样品。

然而，在大多数仪器中，尼柯耳棱镜已被其它类型的偏振器所取代，如宝丽来片和格兰-汤普逊棱镜。

A Nicol prism is a type of polarizer. It is an optical device made from calcite crystal used to convert ordinary light into plane polarized light. It is made in such a way that it eliminates one of the rays by total internal reflection, i.e. the ordinary ray is eliminated and only the extraordinary ray is transmitted through the prism.

It was the first type of polarizing prism, invented in 1828 by William Nicol (1770–1851) of Edinburgh.

Mechanism

The Nicol prism consists of a rhombohedral crystal of Iceland spar (a variety of calcite) that has been cut at an angle of 68° with respect to the crystal axis, cut again diagonally, and then rejoined, using a layer of transparent Canada balsam as a glue.

Unpolarized light ray enters through the side face of the crystal, and is split into two orthogonally polarized, differently directed rays by the birefringence property of calcite. The ordinary ray, or o-ray, experiences a refractive index of no = 1.658 in the calcite and undergoes a total internal reflection at the calcite–glue interface because of its angle of incidence at the glue layer (refractive index n = 1.550) exceeds the critical angle for the interface. It passes out the top side of the upper half of the prism with some refraction. The extraordinary ray, or e-ray, experiences a lower refractive index (ne = 1.486) in the calcite crystal and is not totally reflected at the interface because it strikes the interface at a sub-critical angle. The e-ray merely undergoes a slight refraction, or bending, as it passes through the interface into the lower half of the prism. It finally leaves the prism as a ray of plane-polarized light, undergoing another refraction, as it exits the opposite side of the prism. The two exiting rays have polarizations orthogonal (at right angles) to each other, but the lower, or e-ray, is the more commonly used for further experimentation because it is again traveling in the original horizontal direction, assuming that the calcite prism angles have been properly cut. The direction of the upper ray, or o-ray, is quite different from its original direction because it alone suffers total internal reflection at the glue interface, as well as a final refraction on exit from the upper side of the prism.

Uses

Nicol prisms were once widely used in mineralogical microscopy and polarimetry, and the term "using crossed Nicols" (abbreviated as XN) is still used to refer to the observing of a sample placed between orthogonally oriented polarizers.

In most instruments, however, Nicol prisms have been replaced by other types of polarizers such as polaroid sheets and Glan–Thompson prisms.