In special circumstances, the phase velocity of a wave can exceed the speed of the light, c. However, this doesn’t imply that the signal is propagated faster than c. In the majority of the optical media, the refraction index is greater than the summation of all the wavelengths’, keeping the phase velocity lower than the speed of the light.
Group velocity higher than the speed of the light
During the last decade, different studies tried to show that the group velocity can be higher than the speed of the light. However, they have concluded that, if this were true, we could transmit superluminal pulses and they won’t be able to carry information.
One of these studies was published in the Nature Journal, in 2000. According to the authors, the superlumnial pulses are the result of classic interference mechanisms due to the undulatory nature of the light. In addition, there won’t be information transmitted (signal) at a rate higher than the speed of the light. This would challenge the Einstein’s Theory of Relativity. You can have a look at the complete study in the frame below: