Can light travel at a lower speed compared to its speed in vacuum?
In any transparent medium light travels slower. Its speed V depends on the refractive index N of the medium: V = C/N. The refractive index of air is 1.0003, water of 1.33, and various types of glass - from 1.5 to 1.8. One of the biggest values of the refractive index has a diamond of 2.42.