自然光的定義：
普通光源中，單個原子發出的光波列是偏振的，大量原子獨立發出的光波列的振動方向毫無規則，並且隨時間迅速變化，這種光叫自然光，又稱“天然光”。自然光中，光振動沿光傳播方向對稱分佈，不直接顯示偏振現象的光。天然光源和一般人造光源直接發出的光都是自然光。它包括了垂直於光波傳播方向的所有可能的振動方向，所以不顯示出偏振性。(http://physics.seu.edu.cn/phycourse/Article/ShowArticle.asp?ArticleID=312)
光的偏振 polarization of light
光是一種電磁波，電磁波是橫波。而振動方向和光波前進方向構成的平面叫做振動面，光的振動面只限於某一固定方向的，叫做平面偏振光或線偏振光。光波電矢量振動的空間分佈對於光的傳播方向失去對稱性的現象。只有橫波才能產生偏振現象，故光的偏振是光的波動性的又一例證。在垂直於傳播方向的平面內，包含一切可能方向的橫振動，且平均說來任一方向上具有相同的振幅，這種橫振動對稱於傳播方向的光稱爲自然光（非偏振光）。凡其振動失去這種對稱性的光統稱偏振光。光的偏振可以參考視頻http://physics.seu.edu.cn/phycourse/Article/ShowArticle.asp?ArticleID=311。
偏振光
偏振光是指光矢量的振動方向不變，或具有某種規則地變化的光波。按照其性質,偏振光又可分爲平面偏振光（線偏光）、圓偏振光和橢圓偏振光、部分偏振光幾種。如果光波電矢量的振動方向只侷限在一確定的平面內，則這種偏振光稱爲平面偏振光，若軌跡在傳播過程中爲一直線，故又稱線偏振光。如果光波電矢量隨時間作有規則地改變，即電矢量末端軌跡在垂直軌跡在傳播過程中爲一直線，故又稱線偏振光。如果光波電矢量隨時間作有規則地改變，即電矢量末端軌跡在垂直於傳播方向的平面上呈圓形或橢圓形,則稱爲圓偏振光或橢圓偏振光。如果光波電矢量的振動在傳播過程中只是在某一確定的方向上佔有相對優勢，這種偏振光就稱爲部分偏振光。偏振光的視頻請參考http://physics.seu.edu.cn/phycourse/Article/ShowArticle.asp?ArticleID=313。
百度百科上有偏振光的解釋：http://baike.baidu.com/view/70598.htm。
<script language=JavaScript> </script> 以下摘自：http://plc.cwru.edu/tutorial/enhanced/files/lc/light/light.htm
Light and Polarization
This section will introduce some of the basic concepts that are important in understanding the optical behavior of liquid crystals. This is by no means a complete discussion of the topic; it is only intended to be used in the context of liquid crystal optical behavior. Please refer to Jenkins and White for a detailed treatment.
Light and Polarization
Light can be represented as a transverse electromagnetic wave made up of mutually perpendicular, fluctuating electric and magnetic fields. The left side of the following diagram shows the electric field in the xy plane, the magnetic field in the xz plane and the propagation of the wave in the x direction. The right half shows a line tracing out the electric field vector as it propagates. Traditionally, only the electric field vector is dealt with because the magnetic field component is essentially the same.
This sinusoidally varying electric field can be thought of as a length of rope held by two children at opposite ends. The children begin to displace the ends in such a way that the rope moves in a plane, either up and down, left and right, or at any angle in between.
Ordinary white light is made up of waves that fluctuate at all possible angles. Light is considered to be "linearly polarized" when it contains waves that only fluctuate in one specific plane. It is as if the rope is strung through a picket fence -- the wave can move up and down, but motion is blocked in any other direction. A polarizer is a material that allows only light with a specific angle of vibration to pass through. The direction of fluctuation passed by the polarizer is called the "easy" axis.
If two polarizers are set up in series so that their optical axes are parallel, light passes through both. However, if the axes are set up 90 degrees apart (crossed), the polarized light from the first is extinguished by the second. As the angle rotates from 0 to 90 degrees, the amount of light that is transmitted decreases. This effect is demonstrated in the following diagram. The polarizers are parallel at the top and crossed at the bottom.
Polarized Light
Linear polarization is merely a special case of circularly polarized light. Consider two light waves, one polarized in the YZ plane and the other in the XY plane. If the waves reach their maximum and minimum points at the same time (they are in phase), their vector sum leads to one wave, linearly polarized at 45 degrees. This is shown in the following diagram.
Similarly, if the two waves are 180 degrees out of phase, the resultant is linearly polarized at 45 degrees in the opposite sense.
If the two waves are 90 degrees out of phase (one is at an extremum and the other is at zero), the resulting wave is circularly polarized. In effect, the resultant electric field vector from the sum of the components rotates around the origin as the wave propagates. The following diagram shows the sum of the electric field vectors for two such waves.
The most general case is when the phase difference is at an arbitrary angle (not necessarily 90 or 180 degrees.) This is called elliptical polarization because the electric field vector traces out an ellipse (instead of a line or circle as before.)
These concepts can be rather abstract the first time they are presented. The following simulation allows the user to change the phase shift to an arbitrary value to observe the resultant polarization state.
此外，還有偏振數據庫，不知道這個東東做什麼用的，http://www.sal.wisc.edu/WUPPE/polcats/polcats.html。