Definitions: Rotation of Light
Optical isomers are named because they can rotate a plane of polarized light. Light is plane-polarized if all of the light waves are vibrating in the same, parallel, direction. This is shown below with a single wave (line) below.
If plane-polarized light is rotated 45 degrees to the left (counterclockwise), this is known as levorotatory
, -); this is shown below in the image on the left. If plane-polarized light is rotated 45 degrees to the right (clockwise) this is known as dextrorotatory
, +); this is shown below in the image on the right. Both d
rotations are considered by looking in the direction the light is traveling; in other words, the light is moving away from you, not toward you.