Question

What materials slow light waves down? Does it have to do something with the density? Please help me understand, medals will be given.

Answer 1

The optical density of a material is the result of the tendency of the atoms of a material to maintain the absorbed energy of the light wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. Thus, while a light wave travels through a vacuum at a speed of c= (3.00 x 108 m/s), it travels through a transparent material at speeds less than c. The index of refraction value (n) provides a quantitative expression of the optical density of a given medium. Materials with higher index of refraction values have a tendency to hold onto the absorbed light energy for greater lengths of time before reemitting it to the interatomic void. The more closely that the frequency of the light wave matches the resonant frequency of the electrons of the atoms of a material, the greater the optical density and the greater the index of refraction. A light wave would be slowed down to a greater extent when passing through such a material.

Answer 2

Fine explanation.
Velocity in medium = (speed of light in vacuum)/(index of refraction)

Answer 3

Well said @clarissaisaacs.
I'll add that the speed of light 'in a vacuum' tells you that any and all material will slow light down. The question is by how much.