Isomerism
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I am studying optical isomers. I know that they rotate the plane of polarised light, but I don't know what the plane of polarised light is, and how or why it is rotated.
Ulex says
It would really be best if you could actually see this very curious effect rather than just having it described to you, but I will do what I can!
Light generated by the filament of a bulb, for example, consists of energetic vibrations - light waves. If you look towards such a source of light, your eye receives vibrations in all the possible planes at right angles to the direction of travel of the light waves. If you could actually see the vibrations they would look something like this:
This light is 'unpolarised'.
In this illustration each line represents the end-on view of a vibration.
Now, when light passes through certain crystals or a plastic material called ‘polaroid’ most of these vibrations are stopped, only the ones in a particular direction are allowed through. The end-on view of this would look more like this: 
The light is now said to be ‘plane polarised’.
When plane polarised light is passed through a solution of an optical isomer, the plane of the vibration is rotated, either clockwise (right)
Or anticlockwise (left) 
One of the optical isomers rotates the plane to the right, the other to the left.
I hope this makes some sort of sense! Your final question is much harder to answer: Why does this rotation happen? Frankly, I am not sure that anyone really knows, but what we do know is that the rotation is related to the structure of the isomeric molecules which are optical reflections of each other.
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updated: 15 December 2006
