Doppler Effect: Does Light Obey the Rule?

In summary, the Doppler effect for light works because although the speed of light is constant, the relative speed between the light and a moving object is different from c. This means that the wavelength of light can appear to change as the object moves towards or away from the observer.
  • #1
bart0211
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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
 
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  • #2
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
bart0211 said:
But how can that work for light if the relative speed of the wave compared to the moving object's is constant?
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 [itex]v_s[/itex]. 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.)
 

1. What is the Doppler Effect?

The Doppler Effect is the change in frequency or wavelength of a wave, such as sound or light, due to the relative motion between the source of the wave and the observer.

2. How does the Doppler Effect affect light?

The Doppler Effect affects light in the same way it affects sound. The frequency of light waves will appear to increase as the source moves towards the observer and decrease as the source moves away from the observer.

3. Does light always obey the rules of the Doppler Effect?

No, light does not always obey the rules of the Doppler Effect. It only applies to certain situations, such as when the source of light is moving towards or away from the observer at a high velocity.

4. What is the formula for calculating the Doppler Effect for light?

The formula for calculating the Doppler Effect for light is: f' = f(v/c + 1), where f' is the observed frequency, f is the emitted frequency, v is the relative velocity between the source and observer, and c is the speed of light.

5. How is the Doppler Effect used in astronomy?

The Doppler Effect is used in astronomy to determine the motion and velocity of celestial objects, such as stars and galaxies. By analyzing the shift in frequency of light emitted from these objects, scientists can gather information about their movement and distance from Earth.

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