explain beam polarization

hugo/content/ref/instr/polarized/_index.md

@@ -8,6 +8,16 @@ weight = 60
 Sample magnetization is explained in
 [Sample model > Materials > Magnetization](ref/sample/material/magnetization).
 
-
+Beam polarization is set through
+```python
+beam.setPolarization(polarizer_dir) # for scattering
+scan.setPolarization(polarizer_dir) # for specular and offspec
+```
+The `polarizer_dir` is a Bloch vector, created by
+```
+R3(x, y, z)
+```
+If it is a unit vector, then the beam polarization is perfect.
+A shorter vector indicates imperfect polarization.
 
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