# Light Doppler Effect

1. Aug 20, 2004

### bart0211

Assuming the speed of light is constant from all reference points I don't quite understand why the Doppler Effect works for electromagnetic waves. From my understanding, as far as sound is concerned the object "catches up" with the sound wave, increasing the wave's frequency. But how can that work for light if the relative speed of the wave compared to the moving object's is constant?
-Bart

2. Aug 21, 2004

### Chronos

In a media dependant reference frame, propogation speed of a wave is media dependent. Mach's law. In a vacuum, it must obey the Maxwell equation.

3. Aug 21, 2004

### Staff: Mentor

Welcome to PF!

While it is true that all observers measure the speed of light as a constant (c) with respect to themselves, an observer will measure the "separation rate" between light and a moving object as being different from c.

Let's say the object is moving towards you while emitting light. According to you, the light approaches you at speed c and the object (the light source) approaches you at speed $v_s$. So, from this alone, you can see that the source does "catch up" with the light, thus reducing the apparent wavelength. (To get the correct expression for the light Doppler effect one must also include relativistic effects, e.g., time dilation.)