I am trying to understand the relation between transparency and refraction index. Searched this forum so found and read some topics too. As I undertand, transparency is related with atomic structure and phonons. This is ok. If light beam (or say photons) faraway from phonons (and atoms), they can pass easily. But this seem a very ideal situation. Lets think a flat glass. Glass is transparent to visible light but its refractive index is not equal to vacuum too. So, i think light beam passing one molecule(atom) to other by luminesance. Since every transparent media also has refractive index then some parts of beams passing one molecule to other. Shortly, because of above statements, if a substance is transparent, than light beam passed through phonons and atoms approach is not completely true for me. Glass is transparence for visible light but the "past light beams" are not only the beams without interact phonons and atoms but also the beams from luminesance. So, all transparent substances(with suitable light) 1. İnteract with phonons and atoms. Energy goes to thermal energy and luminesance. 2. some of them doesn't interact with atomic stucture and pass through whenever possible. Conclusion, the only real transparent substance must be vacuum. If refractive index of a transparent(visible light) substance is not equal to 1 then we can not say, the "only past light" coming between phonons, molecules, atoms.