Please answer these questions BRIEFLY :)Is this because of induction? ( see picture)What exactly is meant by mass in this context because from the previous example about the radio station signals it doesn't describe mass but it should be involved I would presume?So an electromagnetic wave involves movement of electric field and magnetic field in perpendicular directions parallel to a moving "wave front"?

In the last several chapters, we've stud- ied various aspects of electric and mag- netic fields. When the fields don't vary with time, such as electric fields pro- duced by charges at rest, or magnetic fields produced by steady currents, we can analyze the electric and magnetic fields independently, without consider- ing interactions between them. But when the fields vary with time, they're no longer independent