Understanding the Doppler Effect of Light: How Speed Impacts Color Perception

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Discussion Overview

The discussion centers on the Doppler effect of light, specifically how the speed of an observer affects the perceived color of light, with a focus on the conditions under which a red light might appear green. Participants explore the mathematical formulation of the Doppler effect and its implications for frequency shifts.

Discussion Character

  • Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant recalls that approaching a red light at high speed could cause it to appear green, prompting a question about the necessary speed for this effect.
  • Another participant provides a formula for calculating the frequency shift due to the Doppler effect, introducing variables for the observer's speed and the speed of light.
  • A subsequent reply expresses appreciation for the provided equation.
  • Another participant points out a potential typo in the equation, suggesting that it incorrectly states the frequency decreases when the source approaches the observer.
  • A later reply acknowledges the typo and proposes a corrected version of the equation, indicating that the signs should be reversed for the case of an approaching source.

Areas of Agreement / Disagreement

There is no consensus on the accuracy of the original equation, as participants disagree on its correctness and propose corrections. The discussion remains unresolved regarding the implications of these corrections for understanding the Doppler effect.

Contextual Notes

The discussion includes unresolved aspects related to the assumptions made in the equation and the conditions under which the Doppler effect is being analyzed. There is also a dependence on the definitions of the variables used in the formula.

cragar
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I remember reading in a book that if we approached a red light
really fast it would appear green to us due to the Doppler effect of light
does anyone know how fast we would need to travel for this to happen.
 
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If you are approaching a light source, then you can compute the shift in frequency of light by

\nu_O=\nu_S\sqrt{\frac{1-v/c}{1+v/c}}

where \nu_O is the frequency seen by the observer, \nu_S is the frequency emitted by the source of the light, c is the speed of light, and v is the speed that you're traveling toward the source.
 
sweet that helps , thank-you
 
glad to help.
 
The equation above seems to have a typo, because the equation says when source approaches the observer the freq decreases , which is wrong.
 
Ah yes there is a typo--for approaching the signs should be reversed

\nu_O=\nu_S\sqrt{\frac{1+v/c}{1-v/c}}
 

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