[al2o3cr]

The "left" or "right" refer to the direction that the chemical (usually in solution) rotates the plane of polarized light passing through it, from the point of view of an observer that the light is traveling towards.

An experimental setup to demonstrate this is a light source, a linear polarizer oriented in some direction (say along the x-axis for definiteness), followed by the sample container, followed by another linear polarizer, followed by a detector.

With nothing in the sample container, the second polarizer is rotated around the light beam axis until no light passes through. This will be very close to a right angle with the first one (try it with polarizing sunglasses :) ). The sample is then added, and the change in angle of the second polarizer required to again allow no light through is observed.