In this book, we discussed dielectric constant \(\epsilon \) of a media. We can write \(\epsilon = \epsilon _0 ( 1 + \chi )\) , where \(\epsilon _0\) is the dielectric constant of vacuum and \(\chi \) is called electric susceptibility of the medium. What is the source of \(\chi \) ? \(\chi \) results in different light velocities in a medium. Why do radio waves go through the wall as in mobile telephones while light does not? Radio waves are not scattered by the medium, light is. What is the source of all beautiful colors around us? In this chapter, we will try to answer some of these questions.