Unpolarized light symmetric about the direction of propagation but polarized light is not?

In my textbook it was written unpolarized light was symmetric about the direction of propagation of light(EM wave)but not so in plane polarized light ....can someone please explain to me as how this is?

1 Answer

Unpolarized light is statistically symmetric about the direction of propagation, because every photon has an equal probably of having its $E$-field in some direction normal to the direction of propagation. Therefore, if you put a polarizer in its path, no matter how you turn it, it will always let the same intensity through.

For linearly polarized light, however, all photons have their $E$-field in the same plane, therefore the intensity changes when you turn the polarizer, i.e. it is not symmetric anymore.

• Here symmetric about the direction of propagation .what does this phrase means?? – user150960 Aug 27 '17 at 15:20
• It means that there is no unique direction of polarization. Every direction normal to the direction of propagation is "equal" in some sense, as explained above, therefore the term symmetric. – noah Aug 27 '17 at 15:25