Q.Explain why an electromagnetic wave is able to travel through empty vacuum, unlike a mechanical wave such as sound, which needs a material medium.

How a mechanical wave like sound works. Sound is a longitudinal PRESSURE wave -- what is actually oscillating is the physical displacement (back-and-forth motion) of real air molecules, each one bumping into its neighbour and passing the disturbance along. Remove the air (a true vacuum) and there is nothing left to do the bumping, so sound simply cannot travel through it at all.

How an electromagnetic wave is fundamentally different. An electromagnetic wave's oscillating quantity is not the displacement of any physical particle -- it is the electric field $\vec{E}$ and magnetic field $\vec{B}$ THEMSELVES, existing at every point of space regardless of whether any matter is present there. As shown in Section 8.3 and Exercise 2, a changing electric field produces a magnetic field, and a changing magnetic field produces an electric field, purely as a consequence of Maxwell's equations -- a relationship that holds in completely empty space just as well as inside any material medium, since it never actually calls on any material particle to do anything. …

Contrast what physically oscillates in a mechanical wave (matter) with what oscillates in an electromagnetic wave (the fi …

• Saying electromagnetic waves 'don't need energy' to travel, rather than identifying correctly that it is the ABSENCE OF A MATERIAL CARRIER, not energy, that distinguishes them from sound. …