Doppler Effect: Does Light Obey the Rule?

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SUMMARY

The discussion clarifies how the Doppler Effect applies to light, emphasizing that while the speed of light remains constant at c for all observers, the perceived frequency changes due to the relative motion of the light source. When an object moves towards an observer while emitting light, the observer perceives a shorter wavelength, effectively increasing the frequency. This phenomenon requires consideration of relativistic effects, such as time dilation, to accurately express the light Doppler effect.

PREREQUISITES
  • Understanding of the Doppler Effect in sound waves
  • Familiarity with the concept of the speed of light (c)
  • Basic knowledge of Maxwell's equations
  • Awareness of relativistic effects, particularly time dilation
NEXT STEPS
  • Study the mathematical formulation of the light Doppler effect
  • Explore the implications of time dilation in special relativity
  • Investigate Maxwell's equations and their relevance to electromagnetic wave propagation
  • Learn about Mach's principle and its impact on wave behavior in different media
USEFUL FOR

Physicists, students of physics, and anyone interested in the principles of wave behavior and relativity, particularly in the context of electromagnetic waves.

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

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