# Doppler effect for light only depends upon the relative motion of the source of sound and observer?

How would doppler effect for light only depend on relative motion of source and observer ?

While studying my text says :

" Doppler effect for sound depends upon the relative motion of source and listener and it also depends upon that which one of these is in motion.

Where is The Doppler effect for light depends only upon the relative motion of the source of sound and observer"

I understand it for the sound but for light I don't get two things written

1. why doppler effect for light depends on relative motion of sound ? and not light ?

2. why would it only depend on relative motion of source and observer?

• "The Doppler effect for light depends only upon the relative motion of the source of sound and observer" that must be a mistake. it should read :"light" Commented Mar 31, 2017 at 10:29
• @Anna V that's what I was confused with Commented Mar 31, 2017 at 11:13

## 1 Answer

For sound which has to travel through a medium you have a reference frame, the medium, relative to which you can measure the velocities of the observer, the source and the sound.
If an observer moves towards a source of sound and measures the speed of sound relative to the observer it will be greater than the speed of sound as measured relative to the medium.

There is no such reference frame for light and the speed of light is constant.
The best you can do in terms of measuring speeds is to measure the speed of the source relative to the observer.

Doppler effect for sound depends upon the relative motion of source and listener and it also depends upon that which one of these is in motion

The motion mentioned here is measured relative to the medium through which the sound is travelling.
So a moving source and a stationary observer means that the source is moving relative to the medium but the observer is not moving relative to the medium.

There is no such medium frame of reference for light.