If speed of light is constant, how does prismatic color shift occur?

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Discussion Overview

The discussion centers around the phenomenon of prismatic color shift and the implications of the speed of light in different media. Participants explore how varying wavelengths and frequencies of light affect its behavior when transitioning between mediums, particularly in relation to refraction and chromatic dispersion. The conversation also touches on the perception of light from distant stars and how it may appear to be white light despite the underlying complexities.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that differing wavelengths of light suggest longer travel paths for certain colors, raising questions about the implications for their arrival at a point of observation.
  • Others explain that in a medium like glass, the speed of light varies with frequency, leading to chromatic dispersion where blue light travels slower than red light.
  • A participant questions whether light from distant sources is always out of phase and expresses confusion about observing a spectrum that appears white from distant stars.
  • Some responses clarify that the speed variation of light applies only in non-vacuum mediums and does not affect starlight traveling through a vacuum.
  • Participants note that starlight is not truly white and can shift towards red due to factors like distance and gravitational effects, referencing Hubble's observations and Einstein's general relativity.
  • There is a discussion about how the geometry of the prism affects light travel, with claims that flat glass can lead to no net bending or color change under certain conditions.
  • One participant raises a question about whether refraction affects the overall speed reading of light as it enters different mediums.
  • Another participant mentions that in a dispersive medium, the index of refraction affects the group velocity of light.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the behavior of light in different mediums, the implications for distant starlight, and the effects of refraction. The discussion remains unresolved with no consensus on several points raised.

Contextual Notes

Some limitations include the dependence on the definitions of speed in different contexts, the unresolved nature of how light behaves at the interface of different media, and the complexities of measuring light from distant sources.

AJHank
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If differing wavelengths explain color variations, would not the longer "zig-zags" of the separate colors for the bluer spectrum portions of visible light suggest that they have traveled longer to get to a discrete point from that of their origin? - Assuming that the component colors of visible light began at the point at the same time - and does frequency play a part in this? And what about the observable bend and shift when a straight object is observed entering water from the air?
 
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When traveling through a medium such as glass, the effective speed of light is slower (compared to its speed in vacuum) and also depends on the frequency. Generally, blue light (higher frequency) travels slower than red light (lower frequency) through a transparent medium such as a glass prism. (This dependence of speed on color is called chromatic dispersion.)

And when light crosses the interface between two media (like between air and glass or air and water), the direction changes according to Snell's law of refraction. The amount of refraction (bending) depends on the relative speed of the light in the two media. Thus, going back to the prism, blue light gets bent more than red, allowing the different colors to be spread out in a spectral pattern.
 
Is light from distant sources then always out of phase?

Thank You for your reply. And you centered me on what was troubling me: if the varying portions of the spectrum are traveling at different speeds, then what exactly are we observing from very distant objects when the entire (or a great portion thereof) spectrum seems to arrive at our point of of observation. Why do stars, for example, appear to be white light to our eyes??
 
AJHank said:
Thank You for your reply. And you centered me on what was troubling me: if the varying portions of the spectrum are traveling at different speeds, then what exactly are we observing from very distant objects when the entire (or a great portion thereof) spectrum seems to arrive at our point of of observation. Why do stars, for example, appear to be white light to our eyes??

Your eyes are not a very good measuring device.

Zz.
 
Just to be sure you understood the answer; the varying portions of the spectrum travel at different speeds only happens in a non-vacuum medium. Thus this doesn’t apply to starlight traveling in a vacuum.

Also when we look closely (and take measurements) we see that starlight is not ‘white’. The futher it came from the more it shifts to the red (Hubble). And if it came from a star surface effected by a very very heavy star with significant time distortion at that surface it is shifted even more red (Einstein GR).

Finally on that “prism” if the surfaces happen to be parallel (flat glass) and the light hits at an angle, the blue light travels a shorter distance in the glass so it comes out sooner and has extra time at full speed to ‘catch-up’ to the rest. Thus no net bend or color change for flat glass.
 
RandallB said:
Just to be sure you understood the answer; the varying portions of the spectrum travel at different speeds only happens in a non-vacuum medium. Thus this doesn’t apply to starlight traveling in a vacuum.
Also when we look closely (and take measurements) we see that starlight is not ‘white’. The futher it came from the more it shifts to the red (Hubble). And if it came from a star surface effected by a very very heavy star with significant time distortion at that surface it is shifted even more red (Einstein GR).
Finally on that “prism” if the surfaces happen to be parallel (flat glass) and the light hits at an angle, the blue light travels a shorter distance in the glass so it comes out sooner and has extra time at full speed to ‘catch-up’ to the rest. Thus no net bend or color change for flat glass.

Additionally the colour of the star is determined by the surface temperature and is pretty well modeled as a black body. Cooler stars are towards the red end of the spectrum and hotter stars the blue.
 
Wouldn't refraction epitomise the speed of light entering different mediums and affect its overall speed reading?
 
DM said:
Wouldn't refraction epitomise the speed of light entering different mediums and affect its overall speed reading?

In a normal dispersive medium, the different index of refraction would affect its GROUP VELOCITY.

Zz.
 

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