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Hi,
The speed of light in vacuum is a fundamental constant, equal to about 2.99 x 108 m/s. In media other than vacuum, light travels at a speed given by (2.99 x 108)/n m/s, where n is the refractive index of the medium. The highest known refractive index is of Diamond, and is equal to aout 2.4. Artificial materials have been reported with indices in the range of 3.5 - 4. So the minimum speed of light in this regard is 2.99 x 108/4 = 7.5 x 107 m/s approx. This is the minimum PHASE velocity of light, and of photons. But there is also a concept of GROUP velocity, and the group velocity of light has been reduced to up to 39 mph in labs using special techniques!! But group velocity is not the speed of individual photons, but of all the photons together. It can be betterunderstood only with the help of quantum mechanics. This kind of light is called "Slow light".
Slow light is a dramatic reduction in the group velocity of light, not the phase velocity. Slow light effects are not due to abnormally large refractive indices, as explained below. A human perceives the amplitude of the sinusoidal disturbance as the brightness of the light and the frequency as the color. If a light is turned on or off at a specific time or otherwise modulated, then the amplitude of the sinusoidal disturbance is also time-dependent. The time-varying amplitude does not propagate at the phase velocity but rather at the group velocity. The group velocity depends not only on the refractive index of the material, but also the way in which the refractive index changes with frequency (i.e. the derivative of refractive index with respect to frequency). Slow light refers to a large reduction in the group velocity of light. If the dispersion relation of the refractive index is such that the index changes rapidly over a small range of frequencies, then the group velocity might be very low, thousands or millions of times less than c, even though the index of refraction is still a typical value (between 1.5 and 3.5 for glasses and semiconductors).
I hope you get the idea....you can read more at http://en.wikipedia.org/wiki/Slow_light
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