# Q & A: speed of light in moving medium

Q:
I know that the speed of light in a vacuum is always measured to be the same regardless of the observer's frame of reference. My question is, does the same apply to the speed of light as it travels through any non-vacuum medium? Say light travels at 0.9c through water, as measured from a stationary observer relative to the water. Will it travel at 0.9c through the water as measured by an observer moving 0.9c relative to the water? Or more generally The speed of light in vacuum is always c, no matter which frame of reference we use. Changing speed of the light source to v has no effect too (i.e., speed of light will not be v+c). When light enter a medium with refractive index n, the speed of light slows down to (c/n). In this case, will we always measure c/n for the speed of the light in the medium regardless of which frame of reference we use? Will moving the light source with a speed of v in the medium affect c/n (i.e., change its speed to c/n+v)?
- tom (age 30)
Singapore
A:
Tom- I've combined your two questions, the more specific and more general forms.

If you're at rest with respect to the medium, the speed of light in it will be c/n regardless of the source. Note, however, that n is different for different frequencies of light.

If you're moving with respect to the medium, then you'll see a different speed. I believe that  this effect was first inferred by Fresnel in the early 19th century and confirmed by an experiment by Fizeau in 1851.

Mike W..

(published on 04/22/2013)