Light in moving media

Requisites Edit

Relativistic addition of velocities

Refractive index

Problem Edit

Light moves through a slowly moving medium with refractive index n. That medium moves with a speed v in parallel with the direction of light. What speed will be measured for that light by a rest observer?

Solution Edit

If we have a medium with refractive index n, the speed of light relative to that medium is c/n.

Using relativistic addition of velocities, we get for the rest observer:


But as  , we can expand that expression in terms of  :



The factor   was known as the Fresnel drag coefficient. It is easily measured with interference experiments.

Generalization Edit