光学活性Optical rotation

平面偏振光可看作左、右旋圆偏振光的叠加。手性介质对两者的折射率不同（圆双折射），两束光通过后产生相位差，合成的偏振面因而旋转。旋光度随波长变化（旋光色散），吸收带附近还出现圆二色性（CD）。

Plane-polarised light is a superposition of left and right circularly polarised light. A chiral medium has different refractive indices for the two (circular birefringence), so a phase difference builds up and the plane rotates. Rotation varies with wavelength (optical rotatory dispersion), and near absorption bands circular dichroism appears.

用旋光仪测定观测旋光角 α，换算为比旋光度 [α] = α/(l·c)，通常在钠 D 线（589 nm）、20 °C 测定，须注明溶剂和浓度。

A polarimeter measures the observed rotation α, converted to specific rotation [α] = α/(l·c), usually at the sodium D line (589 nm) and 20 °C with solvent and concentration stated.

1811 年阿拉戈在石英晶体中发现旋光；1815 年毕奥发现松节油、糖溶液等液体也有旋光性，说明旋光性源于分子本身；1848 年巴斯德把旋光性与分子的手性联系起来；1874 年范特霍夫与勒贝尔提出四面体碳给出解释。

Arago found rotation in quartz in 1811; Biot showed in 1815 that liquids such as turpentine and sugar solutions rotate light, so the effect is molecular; Pasteur linked it to molecular handedness in 1848; van ’t Hoff and Le Bel explained it with the tetrahedral carbon in 1874.