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

  1. Dec 16, 2005 #1
    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?
    Last edited: Dec 16, 2005
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  3. Dec 16, 2005 #2

    Doc Al

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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.
  4. Dec 17, 2005 #3
    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 travelling 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??
  5. Dec 17, 2005 #4


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    Your eyes are not a very good measuring device.

  6. Dec 17, 2005 #5
    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.
  7. Dec 22, 2005 #6
    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.
  8. Dec 23, 2005 #7


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    Wouldn't refraction epitomise the speed of light entering different mediums and affect its overall speed reading?
  9. Dec 23, 2005 #8


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

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