Question

How does density of the medium affect the speed of light?

Answer 1

When light enters materials, its energy is absorbed. In the case of transparent materials, this energy is quickly re-radiated. However, this absorption and re-radiation introduces a delay. As light propagates through dielectric material it undergoes continuous absorption and re-radiation. Therefore the speed of light in a medium is said to be less than c, which should be read as the speed of energy propagation at the macroscopic level. At an atomic level, electromagnetic waves always travel at c in the empty space between atoms. Two factors influence this slowing: stronger absorption leading to shorter path length between each re-radiation cycle, and longer delays. The slowing is therefore the result of these two factors. The refractive index of a transparent material is defined as the ratio of c to the speed of light v in the material. Larger indexes of refraction indicate smaller speeds. The refractive index of a material may depend on the light's frequency, intensity, polarization, or direction of propagation. In many cases, though, it can be treated as a material-dependent constant. The refractive index of air is approximately 1.0003. Denser media, such as water and glass, have refractive indexes of around 1.3 and 1.5 respectively for visible light.Diamond has a refractive index of about 2.4