  We calculate the linear magneto-optical Kerr rotation angle for CeSb in the
near-infrared spectral range. Using an exact formula for large Kerr rotation
angles and a simplified electronic structure of CeSb we find at $\hbar \omega =
0.46 eV$ a Kerr rotation of 90 degrees which then for decreasing $\omega $
jumps to -90 degrees in very good agreement with recent experimental
observations. We identify the general origin of possible 90 degree polarization
rotations from mainly optical properties and discuss its relation to the
magnetic moments and magnetic dichroism of the material.
