Have you noticed that the left hand is the mirror image of the right hand, but they cannot be superimposed? That's also true for some molecules containing carbon atoms. In the image above, the molecule on the left cannot be superimposed on the one on the right.
Such left-handed and right-handed molecules rotate the plane of polarization of light in opposite directions. To find out more about polarized light and this important effect, click here. Also, you can learn how Louis Pasteur explained it.
