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  1. Sep 25, 2005 #1
    The speed of light is not always the same but the speed of light does not alter regardless of the speed of the source of the illumination?
  2. jcsd
  3. Sep 25, 2005 #2

    speed of light is always the same i think

    you are really taking advantage of your new membership
  4. Sep 25, 2005 #3
    The speed of light in a vacuum is constant. When light passes through a more optically dense medium such as water or air, it travels slower.

    If you are travelling through a vacuum towards a light source such as a star, you will measure the speed of this light to be 299,792,458 metres per second (this is the speed of light in a vacuum). You would also measure this same speed of light if you were stationary relative the the star or if you were moving away from it. This is the second postulate of Einstein's Special Theory of Relativity: the speed of light in a vacuum is independent of the motion of all observers, and will therefore always have the same value.
  5. Sep 25, 2005 #4


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    Staff: Mentor

    Clarification: light traveling through a medium appears to travel slower.
  6. Sep 26, 2005 #5


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    visible light and other E&M radiation is the result of a changing E field cause a B field which is changing causing a changing E field which causes a changing B field, ...

    now (please allow me to anthropomorphize), once these changing E and B fields are detached from the source of illumination and are then propagating in a vacuum free of the source, how are these changing fields going to know or care what the speed of the source is?

    for sound, in which air (or water for underwater acoustics) is a medium that the sound waves travel in, when the medium moves it affects the propagation of the sound. sound propagating downwind measures faster to sound propagating upwind.

    but, unlike sound, light propagates in a vacuum. and we can't tell the difference between a "moving" vacuum and a "stationary" vacuum. such a difference is meaningless and if that is the case, then the speed of the very same beam of light has to be the same for two observers moving relative to each other. for sound, the observer moving relative to the medium (air, in this case) will measure the speed of the same burst of sound waves as different than an observer that is moving along with the medium.
    Last edited: Sep 26, 2005
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