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Question:
Assume the follow condition: A Prism in a vacuum Now shot a ray of light through the prism. We know through observation that the light will slow relative to the refractive index. However, when the ray of light leaves the prism it speed back up to 3.0x 10^8 m/s when no additional energy is added to the system. How can this be? I know that the conservation of energy must be true, so what am I missing?



Replies:
A photon is massless and hence its energy cannot be equated to mass*(velocity*velocity)/2. Instead it is described by Planck's constant times the frequency. The frequency (and hence the energy) does not change when the light goes from vacuum into glass.

Greg Bradburn



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