Photon with spin 1. But it only has two projection along z. Why?

[xfshi2000]

Hi all:
I am confused about why photon only has two projection along the z-direction. This confusion came from what I read in shankar "printciple of quantum mechanics". In the chapter of field quantization, he explain that because photon must satisfy transverse condition. wave function component along propagation direction is zero. if it is non-zero, it must be remains invariant under rotation (Jz). that is to say, Sz=0 projection is not existed. I don't understand these words meaning. How did he get this conclusion? thanks.
xfshi

 

[DrClaude]

Think of it in terms of polarization. The polarization of the EM wave is in the plane perpendicular to the direction of propagation. Therefore, there is no polarization component along the direction of propagation. For a photon, the latter would corresponds to $S_z = 0$.