regarding speed of light

In the case of an electromagnetic wave, the speed of the wave depends upon the optical density of that material.  The optical density of a material relates to the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. The more optically dense which a material is, the slower that a wave will move through the material.

      optical density = 3 x 10⁸/ v_material