  A misconception that non-chiral molecules have no optical activity at all is
widespread. However, at molecular level even non-chiral molecules have optical
activity. Optical activity of a non-chiral molecule is canceled out by that of
another molecule in its mirror image in normal liquids. We describe the
canceling mechanism by using mirror-symmetry of physical laws without resorting
to detailed formulas. The description will be helpful for overcoming the
misconception. Optical activity can be understood from the opposite viewpoint
by the description. Aligned non-chiral molecules have optical activity.
