# Classical optics: Lambda-quarter plates

Niles
Hi

Say I have a magnetic field point in some well-known direction, and I have linearly polarized light incident in that direction. I want to convert my π-polarization to σ-polarization, so I find my lambda-quarter plate.

However, how do I know whether it produce σ+ or σ-? Do they come with a manual, or do people calibrate them by themselves? I'm not sure how I would do this.

Best,
Niles.

I'm confused by your question- first, the magnetic field will not point along the polarization direction, the polarization is defined by the electric field. Second, to rotate linearly polarized light (if π- and σ- refer to p- and s-polarized) you would use a 1/2 wave plate, not a 1/4-wave plate.

Mentor
lambda/4 wave plates should have some indication where the fast and the slow axes are. The orientation relative to the (linear) polarization of incoming light will determine how the polarization afterwards looks like.

Niles
I'm confused by your question- first, the magnetic field will not point along the polarization direction, the polarization is defined by the electric field. Second, to rotate linearly polarized light (if π- and σ- refer to p- and s-polarized) you would use a 1/2 wave plate, not a 1/4-wave plate.
Hi

What I meant was tbat B points along some direction I call z. My laser propagates along z too, and it is linearly polarized perpendicular to z. I want to convert this linear polarization to σ+ (wrt. to z).

lambda/4 wave plates should have some indication where the fast and the slow axes are. The orientation relative to the (linear) polarization of incoming light will determine how the polarization afterwards looks like.
Hi

Thanks for that.