This chapter covers electromagnetic wavesElectromagnetic wave. The general solution of the wave equationWave equation is composed of waves propagating forward and backward with light speedLight speed. It is shown that the electric fieldElectric field and the magnetic flux densityMagnetic flux density are perpendicular to each other and are perpendicular to the propagation direction. We discuss the refraction and reflection of a plane electromagnetic waveElectromagnetic wave on an interface between two substances with different material constants using the boundary conditions. The Poynting vectorPoynting vector of the electromagnetic waveElectromagnetic wave has a magnitude equal to the electromagnetic energy density multiplied by the light speedLight speed and points in the propagation direction. The propagation of an electromagnetic waveElectromagnetic wave in a wave guideWave guide is also discussed, and we learn about stable standing modes in the wave guideWave guide.