Definitions: Rotation of Light
Enantiomers rotate light by equal amounts, but in opposite directions. (One enantiomer is the d enantiomer and the other enantiomer is the l enantiomer.) Experiments or calculations can be used to determine which way a molecule will rotate a plane of polarized light.
For example, consider the following molecules.
These molecules are enantiomers of glyceraldehyde. (Check for yourself that they are mirror images and non-superimposable.) The molecule on the left is the l
enantiomer of glyceraldehyde, and the molecule on the right is the d
enantiomer of glyceraldehyde. Therefore, these molecules will rotate light by the same amount, but in different directions.