Why optically active molecules rotate light?

[Trave11er]

Say, we have a solution of pure form of R-alanine. Say, the molecule has it's COOH group directed along positive y-axis and rotates the z-polarized light clockwise - the question is why the same molecule with it's COOH group in negative y direction won't do the opposite.

 

[DrDu]

Consider a simpler example, a helix. How does a helix look like when you rotate it by 180 deg?
An example of a chiral molecule with a helical structure are the helicenes:
http://en.wikipedia.org/wiki/Helicene

 

[Trave11er]

Fascinating :). Thank you for the reply, DrDu.