If one is clever, there can indeed be an observable effect on light in the presence of magnetic fields. The Faraday effect is one example. In this effect, the plane of polarization (i.e., the direction the electric & magnetic fields point) can actually be rotated.

If one is clever, there can indeed be an observable effect on light in the presence of magnetic fields while in a medium. The Faraday effect is one example. In this effect, the plane of polarization (i.e., the direction the electric & magnetic fields point) can actually be rotated.