help~~chem...optically active

A chiral carbon atom (also known as asymmetric carbon atom) is a carbon atom with four different groups (or atoms) attached to it in an organic molecule.

If a molecule has a chiral atom, it and the molecule of its mirror image are two isomers called enantiomers. Each enantiomer can rotate the plane of polarization of plane polarized light, and this is said to be optically active. Rotations of plane-polarized light by the pure samples of enantiomers are equal in magnitude but opposite in sign (but all other physical properties of the two enantiomers are identical).

A racemic mixture (or racemate) is a 50:50 mixture of a pair of enantiomers. A racemic mixture shows no rotation of the plane-polarized light, because their optical activities cancel each other.

如果有 chiral，唔一定係 optiically active。

因為如果個 sample 入面o既 chiral compound，向左旋光性同埋向右旋光性o既比例係一樣的話---即個 sample 入面，兩者以 1:1 o既比例混合o係一起，就會完全失去旋光性，即係會變成 optically inactive。

(可以咁諗，optically active 係指個 sample 對旋光性o既旋轉幅度，咁如果個 sample 度，向左轉 30 度同向右轉 30 度o既比例一樣，咁outcome 即係 +30 + (-30) = 0 (分別將兩個方向當成正負)
0 的話，即係唔會偏轉，所以變成o左 optically inactive。)