# Circular Polarization

As one observes an clockwise (cw) circular polarized electromagnetic wave which is reflected off a denser medium or metal interface it is changed to a counter-clockwise (CCW) polarized em wave plus a phase of PI. A counter clockwise polarized wave which is reflected off a lower dense medium interface only get an additional phase of PI.

1. This means the reflected wave always has the opposite polarization?
2. But ONLY because the reference coordinate system is changed (since by definition of polarization the wave always travels away from the observer) and NOT because of some phase which is added only to one or the other linear polarization axis?
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## 1 Answer

The important change is not the change of the reference frame but the change of the momentum of the photons or the direction of the electromagnetic radiation.

If two waves move in different directions, calling their polarizations "the same" or "different" is somewhat similar to comparing apples and oranges. Polarizations are a fixed list with "well-defined identities" only for a single predetermined direction of motion.

If the reflected wave is moving exactly oppositely, the lists of polarization vectors are the same for both directions but it's still a matter of terminology which polarizations are called "the same" and which of them are "the opposite ones".

Waves hitting the metal directly change right-handed polarization to left-handed and vice versa. This follows from the angular momentum conservation: left-handed and right-handed mean the opposite values of the angular momentum if the momentum has the opposite sign for the initial and final waves. And the angular momentum around the axis of direction of motion is conserved due to the rotational symmetry of the environment (including the boundary of the medium) and the laws of physics.

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