How Does Light's Speed Remain Constant in Moving Reference Frames?

[gmsss]

Homework Statement

We send a beam of light, traveling at the speed of light c, toward a distant galaxy, which is receding from us with a speed of 0.90 c. Observers in the galaxy see the light coming toward them with a speed of ?

Homework Equations

cant say

The Attempt at a Solution

i think answer is "c" but don't know exact principle behind it
i tried "Doppler's effect in light" but that tell us about apparent frequency
and i don't know how to start now

 

[Cherrybawls]

Well, I won't pretend to be an expert in this but I believe you are right it should be "c" and I think it has to do with the idea that the speed of light is constant in all reference frames. I think Einstein came up with the idea as the basis for special relativity but someone else might like to come on here and correct me

 

[fluidistic]

I agree with Cherrybawls. I'm still no expert but I'm pretty sure of the answer. It's a postulate of Special Relativity.
More interesting would have been the question if they asked for the frequency of light, in which case you'd have to apply the relativistic Doppler effect.

 

[zorro]

Homework Statement

We send a beam of light, traveling at the speed of light c, toward a distant galaxy, which is receding from us with a speed of 0.90 c. Observers in the galaxy see the light coming toward them with a speed of ?

This is what I think-
The galaxy is moving at a constant speed of 0.90c w.r.t earth. So it is an inertial frame of reference. Whatever is the speed of light on Earth will be the same in that galaxy too.