Re: How can light be circularly polarized

The term circularly polarized is referring to a view of the
resultant amplitude vector as seen by an observer in the line of
propagation of the wave.  The resultant amplitude remains constant
(radius) and circulates clockwise.
If you are familar with birefringence then you
know that what is called a 1/4 wave plate will produce a phase difference
of 90 degrees in two plane-polarized waves.  This can be done by
artificially selecting the desired thickness of the material.  In outer
space (which is not a vacuum) the interstellar medium can provide phase
differences.
Then the waves propogate in free space with this phase difference they
have acquired. 

http://abalone.cwru.edu/tutorial/enhanced/files/lc/light/light.htm