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I was on a ship at night travelling parallel to the shore watching a lighthouse beam hitting the concrete sea wall. As the beam hit the wall at ninety degrees and continued to rotate I saw what appeared to be the light splitting into its spectrum and streaking away at incredible speed. Was I seeing the actual speed of light as the beam's direction changed?

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Could you describe or post a shematics of the exact geometry of the problem, and how you perceived that splitting (like a rainbow I gess, but oriented in which way?) – Nicolas Oct 2 '13 at 16:22
you always see actual speed of light – Dimensionless Oct 2 '13 at 16:22
@Akash err... what do you mean by that? To me, light looks pretty much instantaneous in most cases... – Nicolas Oct 2 '13 at 16:26
yeah i mean that only change in speed og light is not notted by our eyes – Dimensionless Oct 3 '13 at 2:48

Without a more accurate description I can't say for sure but you've probably seen the difference in speed of light (in some medium, perhaps the lighthouse glass) between different colors. That's also known as dispersion (in wave physics) or chromatic aberration in optics.

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