Dextro-rotatory and laevo-rotatory substances are types of optically active compounds that rotate the plane of polarized light in opposite directions.
Dextro-rotatory substances rotate the plane of polarized light to the right or clockwise, while laevo-rotatory substances rotate the plane of polarized light to the left or counterclockwise. The amount of rotation depends on the specific compound and the wavelength of the light.
This property of rotating the plane of polarized light is due to the three-dimensional arrangement of atoms or groups of atoms in the molecule. Optically active compounds have a chiral center, which is a carbon atom that is attached to four different groups. The arrangement of these groups determines the direction and degree of rotation of the plane of polarized light.
Examples of dextro-rotatory substances include glucose and sucrose, while examples of laevo-rotatory substances include fructose and tartaric acid.