Polarization by Absorption: Dichroism

Polarization by Absorption

由吸收产生的偏振

A number of crystalline materials absorb more light in one incident plane than another, so that light progressing through the material become more and more polarized as they proceed. This anisotropy in absorption is called dichroism. There are several naturally occurring dichroic materials, and the commercial material polaroid also polarizes by selective absorption.

一些晶体材料在某一入射平面中吸收更多的光，而在另一平面中吸收较少，因此光通过材料时会逐渐变得更为偏振。这种吸收的各向异性称为双折射。存在几种天然的双折射材料，商业用的偏振片也通过选择性吸收来实现偏振。

Dichroic Materials

双色材料

Materials which have different absorption for perpendicular incident planes for light are said to be dichroic. The mineral tourmaline is the best known of natural materials. Tourmaline refers to a class of boron silicates. A tourmaline crystal has a unique optic axis, and any electric field vector which is perpendicular to that axis is strongly absorbed.

对光的垂直入射平面具有不同吸收能力的材料被称为双色的。已知的天然材料中， tourmaline 是最著名的。 tourmaline 指的是一类含硼硅酸盐。一个 tourmaline 晶体具有一个独特的光学轴，任何与该轴垂直的电场矢量都会被强烈吸收。

Polaroid is strongly dichroic and therefore an effective polarizer. If the transmission axes of ideal polarizers are perpendicular, no light is transmitted. The light tranmitted at other angles follows the Law of Malus.

偏振片是一种强双折射材料，因此是一个有效的偏振器。如果理想偏振器的透射轴相互垂直，就没有光被透射。在其他角度透射的光遵循马吕斯定律。

Polaroid Material

偏振片

Polaroid is the trade name for the most commonly used dichroic material. It selectively absorbs light from one plane, typically transmitting less than 1% through a sheet of polaroid. It may transmit more than 80% of light in the perpendicular plane. The word "polaroid" usually refers to polaroid H-sheet, which is a sheet of iodine-impregnated polyvinyl alcohol. A sheet of polyvinyl alcohol is heated and stretched in one direction while softened, which has the effect of aligning the long polymeric molecules in the direction of stretch. When dipped in iodine, the iodine atoms attach themselves to the aligned chains. The iodine atoms provide electrons which can move easily along the aligned chains, but not perpendicular to them. Light waves with electric fields parallel to these chains are strongly absorbed because of the dissipative effects of the electron motion in the chains. The direction perpendicular to the polyvinyl alcohol chains is the "pass" direction since the electrons cannot move freely to absorb energy.

偏振片是目前最常用的双折射材料的商标名称。它能选择性地吸收某一平面的光，通常通过一片偏振片只能透射不到1%的光。在垂直于该平面的方向上，它可能透射超过80%的光。'偏振片'这个词通常指的是偏振片H片，即浸有碘的聚乙烯醇片。将聚乙烯醇片在软化状态下沿一个方向加热并拉伸，使长链聚合物分子沿拉伸方向对齐。当将其浸入碘中时，碘原子会附着在对齐的链上。碘原子提供可以沿对齐链自由移动的电子，但不能垂直于链移动。由于链中电子运动的耗散效应，电场方向平行于这些链的光波会被强烈吸收。垂直于聚乙烯醇链的方向是'通过'方向，因为电子不能自由移动以吸收能量。

Polarization by Absorption

Dichroic Materials

Polaroid Material

Polaroid Sunglasses